Project Victim of the Week

[Albacore]

Counter

Jirachi @ Leftovers
Ability: Serene Grace
EVs: 252 HP / 4 Def / 252 SpD
Careful Nature
- Iron Head
- Body Slam
- Wish
- Protect

252 SpA Diancie Earth Power vs. 252 HP / 252+ SpD Jirachi: 166-196 (41 - 48.5%) -- guaranteed 3HKO after Leftovers recovery

and I don't think I need to tell you Iron Head OHKOs back. Thanks to Wish+Protect, Jirachi can get its HP back up, so this is a pretty solid counter all things considered.

 

[FullLifeGames]

Counter:

Venusaur @ Venusaurite
Ability: Chlorophyll
EVs: 232 HP / 28 Def / 244 SpA / 4 SpD
Modest Nature
- Giga Drain
- Sludge Bomb
- Hidden Power [Fire]
- Synthesis

With the release of ORAS, there were some new Megas which could break Mega Venusaur, but also some were introduced which Mega Venusaur counters, like Mega Diancie!
Mega Venusaur can easily come in on any move Mega Diancie throws at it and kill it with a Giga Drain.

Relevant Calcs

4 Atk Diancie Diamond Storm vs. 232 HP / 28 Def Venusaur: 184-217 (51.2 - 60.4%) -- guaranteed 2HKO
252 SpA Diancie Earth Power vs. 232 HP / 4 SpD Venusaur: 114-135 (31.7 - 37.6%) -- 90.7% chance to 3HKO
252 SpA Diancie Moonblast vs. 232 HP / 4 SpD Venusaur: 90-107 (25 - 29.8%) -- guaranteed 4HKO
252 SpA Diancie Hidden Power Fire vs. 232 HP / 4 SpD Venusaur: 154-182 (42.8 - 50.6%) -- 2.3% chance to 2HKO

4 Atk Diancie Diamond Storm vs. 232 HP / 28 Def Mega Venusaur: 133-157 (37 - 43.7%) -- guaranteed 3HKO
252 SpA Diancie Earth Power vs. 232 HP / 4 SpD Mega Venusaur: 98-116 (27.2 - 32.3%) -- guaranteed 4HKO
252 SpA Diancie Moonblast vs. 232 HP / 4 SpD Mega Venusaur: 77-91 (21.4 - 25.3%) -- 0.2% chance to 4HKO
252 SpA Diancie Hidden Power Fire vs. 232 HP / 4 SpD Thick Fat Mega Venusaur: 68-80 (18.9 - 22.2%) -- possible 5HKO

244+ SpA Mega Venusaur Giga Drain vs. 0 HP / 0- SpD Diancie: 264-312 (109.5 - 129.4%) -- guaranteed OHKO

 

[paper dreams]

Hard Counter

Bronzong @ Leftovers
Ability: Levitate
EVs: 252 HP / 4 Atk / 252 SpD
Sassy Nature
IVs: 0 Spe
- Stealth Rock
- Gyro Ball
- Earthquake
- Toxic / Hidden Power Ice

M-Diancie can barely come close to even scraping the surface of this bell! Even with everything in it's arsenal Bronzong hard walls it, comes in on any switch and takes it out with ease. Gyro Ball is the mandatory stab move, Earthquake and Hidden Power Ice hit Pokémon switching out from Diancie and nails them, Toxic and SR are some useful status moves that help out the team by weakening down the opposing team one by one.

Calculations:
4 Atk Diancie Diamond Storm vs. 252 HP / 0 Def Bronzong: 72-84 (21.3 - 24.8%)
252 SpA Diancie Moonblast vs. 252 HP / 252+ SpD Bronzong: 59-69 (17.4 - 20.4%)
252 SpA Diancie Hidden Power Fire vs. 252 HP / 252+ SpD Bronzong: 102-120 (30.1 - 35.5%)
4 Atk Bronzong Gyro Ball (96 BP) vs. 0 HP / 0 Def Diancie: 348-412 (144.3 - 170.9%)

 

[blinkie]

reserving bronzong
Edit: Greninja'd
Reserving Magnezone

 

[looiiyut]

Week 31:
Thanks for all the Lopunny checks/counters. I haven't finished going through them yet, but I'll post this week's mon anyway. Congrats to looiiyut2 for making it into the archive!

Diancie @ Diancite
Ability: Clear Body
EVs: 4 Atk / 252 SpA / 252 Spe
Naive Nature
- Moonblast
- Diamond Storm
- Earth Power
- Hidden Power Fire

edit: also a cool sprite for diancie (mega) would be awesome!​

Thanks :]

 

[The TurtleLord]

reserving hippowdon

 

[firehusky]

Landorus-Therian
Check

Landorus-Therian @ Choice Scarf
Ability: Intimidate
EVs: 252 Atk / 24 Def / 232 Spe
Jolly Nature
- Earthquake
- U-turn
- Knock Off / Explosion
- Stone Edge

Landorus-Therian has a really high usage in ORAS, and for good reasons. It's the best scarfer in the game, has a great ability allowing it to counter and check most physical attackers, and is a great choice for any team in need of a pivot. It can also check mega diancie, a very dangerous mixed attacker, that gained many new buffs in ORAS. Earthquake is main STAB, u-turn can gain momentum, knock off beats ghost types and is useful for knocking off items, explosion can also be used to gain more momentum for a free switch in. Stone edge is the last move to hit flying types.

-1 4 Atk Diancie Diamond Storm vs. 0 HP / 24 Def Landorus-T: 117-138 (36.6 - 43.2%) -- guaranteed 3HKO
252 SpA Diancie Moonblast vs. 0 HP / 0 SpD Landorus-T: 219-258 (68.6 - 80.8%) -- guaranteed 2HKO
252 SpA Diancie Hidden Power Fire vs. 0 HP / 0 SpD Landorus-T: 92-109 (28.8 - 34.1%) -- 1.9% chance to 3HKO
252 Atk Landorus-T Earthquake vs. 0 HP / 0 Def Diancie: 326-386 (135.2 - 160.1%) -- guaranteed OHKO
doge is also immune to earthpower, so basically it can safely come in on 3 out of 4 attacks from mega diancie, which is pretty decent.

If lando-t has taken too much prior damage, it will die from a moonblast, so that is why it is classified as a check, because it also does not have reliable recovery.

 

[Froggyboy]

reserving rotom-w

 

[Inspirited]

name: Milotic

check

Milotic @ Leftovers
Ability: Marvel Scale
EVs: 248 HP / 192 Def / 68 SpD
Bold nature
IVs: 0 Atk
- Scald
- Ice Beam
- Recover
- Toxic

The SpA drops from Moonblast make Milotic not a counter, but it is still a solid check. It 2HKOes Mega Diancie with Scald where as any attack from Diancie will only 3HKO after Stealth Rock damage.

calcs

4 Atk Diancie Diamond Storm vs. 248 HP / 192+ Def Milotic: 144-171 (36.6 - 43.5%) -- 99.4% chance to 3HKO after Leftovers recovery
252 SpA Diancie Moonblast vs. 248 HP / 68 SpD Milotic: 142-168 (36.1 - 42.7%) -- 95.2% chance to 3HKO after Leftovers recovery

0 SpA Milotic Scald vs. 0 HP / 0- SpD Diancie: 176-210 (73 - 87.1%) -- guaranteed 2HKO

 

[Random Passerby]

Reserving Mega Aggron as a check.

 

[alexwolf]

Counter

Tangrowth @ Assault Vest
Ability: Regenerator
EVs: 252 HP / 4 SpA / 252 SpD
Sassy Nature
- Leaf Storm
- Hidden Power Ice
- Knock Off
- Earthquake

Tangrowth avoids the 2HKO from all of Mega Diancie's moves even after SR, and OHKOes it back. Other Pokemon / strategies this set checks or counters include Keldeo, Garchomp, Landorus-T, Landorus, Thundurus, Mega Gyarados, Excadrill, Rotom-W, Mega Manectric, Mega Sceptile, Magnezone, Latios, Latias, Manaphy, and rain teams. Hidden Power Ice is for Mega Salamence switch-ins, and for Garchomp, Lando-T, Thundurus, and Mega Sceptile. Knock Off is for Lati@s and a nice move to spam if the opponents MEvo doesn't like facing Tangrowth, and Earthquake hits common switch-ins such as Heatran, Mega Charizard X, and Mega Metagross.

 

[Teeny Victory]

Ok, seeing as my other post was deleted, can I reserve Celebi?

 

[Aquaslash]

name: Milotic

check

Milotic @ Leftovers
Ability: Marvel Scale
EVs: 248 HP / 192 Def / 68 SpD
Bold nature
IVs: 0 Atk
- Scald
- Ice Beam
- Recover
- Toxic

The SpA drops from Moonblast make Milotic not a counter, but it is still a solid check. It 2HKOes Mega Diancie with Scald where as any attack from Diancie will only 3HKO after Stealth Rock damage.

calcs

4 Atk Diancie Diamond Storm vs. 248 HP / 192+ Def Milotic: 144-171 (36.6 - 43.5%) -- 99.4% chance to 3HKO after Leftovers recovery
252 SpA Diancie Moonblast vs. 248 HP / 68 SpD Milotic: 142-168 (36.1 - 42.7%) -- 95.2% chance to 3HKO after Leftovers recovery

0 SpA Milotic Scald vs. 0 HP / 0- SpD Diancie: 176-210 (73 - 87.1%) -- guaranteed 2HKO

Run competitve instead of Marvel Scale and then you counter.

 

[ArchPhantom]

EDIT: This post was originally Celebi but after seeing Teeny Victory's post late-game (heh), I would feel like a jerk if it stayed. Therefore, I will present you with another counter to Mega Diancie:

Original Post

Celebi @ Leftovers
Ability: Natural Cure
EVs: 252 HP / 220 SpD / 36 Spd
Calm Nature
-Giga Drain
-Baton Pass
-Recover
-Nasty Plot / Perish Song / Filler

Celebi is not threatened by M-Diancie whatsoever, as the rock fairys' STAB moves avoid to 2HKO Celebi even after rocks. Celebi can then proceed to 2HKO M-Diancie with Giga Drain.

This Celebi has other uses too, like walling Keldeo and Rotom-Wash, countering dangerous Calm Mind users, such as Mega Slowbro and Suicune, as well as luring in Pursuit users/Dark types to then Baton Pass out, giving you momentum. It can even pass a Nasty Plot boost to one of its teammates.

Calcs:

M-Diancie attacking Celebi:

252 SpA Diancie Earth Power vs. 252 HP / 220+ SpD Celebi: 42-50 (10.3 - 12.3%) -- possibly the worst move ever

4 Atk Diancie Diamond Storm vs. 252 HP / 0 Def Celebi: 118-139 (29.2 - 34.4%) -- 5.4% chance to 3HKO after Stealth Rock and Leftovers recovery

252 SpA Diancie Moonblast vs. 252 HP / 220+ SpD Celebi: 135-159 (33.4 - 39.3%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery

252 SpA Diancie Hidden Power Fire vs. 252 HP / 220+ SpD Celebi: 112-134 (27.7 - 33.1%) -- guaranteed 4HKO after Stealth Rock and Leftovers recovery

Celebi attacking M-Diancie:

0 SpA Celebi Giga Drain vs. 0 HP / 0- SpD Diancie: 168-198 (69.7 - 82.1%) -- guaranteed 2HKO

Counter:

Amoonguss @ Black Sludge
Ability: Regenerator
EVs: 236 HP / 200 Def / 72 SpD
Bold Nature
-Giga Drain
-Spore
-Clear Smog / Filler
-Hidden Power Fire / Foul Play / Filler

Amoonguss is a very good counter to four-attacks Mega Diancie, coming in on any of Diancies' attacks, avoiding the 2HKO even after rocks, and retaliating with Giga Drain, which is a 2HKO. Not being limited to walling rock brides, Amoonguss can also provide its team with the ever-useful Spore, an amazing defensive typing, pony-walling, fairy-walling (bar Mega Gardevoir), and stat-boost removal.

Also, having a switch-in to water and grass types is always appreciated.

Calcs:

Rock Bride Vs. Shroom:

252 SpA Diancie Earth Power vs. 236 HP / 72 SpD Amoonguss: 127-150 (29.6 - 35%) -- 19.2% chance to 3HKO after Stealth Rock and Black Sludge recovery

4 Atk Diancie Diamond Storm vs. 236 HP / 200+ Def Amoonguss: 154-183 (35.9 - 42.7%) -- guaranteed 3HKO after Stealth Rock and Black Sludge recovery

252 SpA Diancie Moonblast vs. 236 HP / 72 SpD Amoonguss: 100-118 (23.3 - 27.5%) -- 6.3% chance to 4HKO after Stealth Rock and Black Sludge recovery

252 SpA Diancie Hidden Power Fire vs. 236 HP / 72 SpD Amoonguss: 170-200 (39.7 - 46.7%) -- guaranteed 3HKO after Stealth Rock and Black Sludge recovery

Shroom Vs. Rock Bride:

0 SpA Amoonguss Giga Drain vs. 0 HP / 0- SpD Diancie: 146-174 (60.5 - 72.1%) -- guaranteed 2HKO after Leftovers recovery

 

[Behexenhaus]

Check

Excadrill @ Air Balloon
Ability: Sand Rush
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Earthquake
- Iron Head
- Rock Slide
- Rapid Spin / Swords Dance

Air Balloon Sand Rush Excadrill is an unexpected check to Mega Diancie. The caveat is that it requires sand from Ttar, so I'll include some calcs on that for completion's sake. Ideally, Mega Diancie would try Earth Power or Moonblast to try to take down Ttar, which is not a problem when switching in Exca. In sand, he outspeeds and OHKOs Mega Diancie easily with either EQ or Iron Head, while Diancie is tempted to stay in to KO him with Earth Power. Sand offense is pretty well known for being able to break through many types of teams in general, and even Mega Diancie does not stop it. Switching into a predicted HP Fire is a bummer, though, and could prevent Exca from ripping through the rest of the opposing team. On another note, Assault Vest Excadrill makes for another check, although it is outsped and nearly 2HKOed by Earth Power (40.4 - 48%), so the Sand Rush set is preferred.

calcs

252 SpA Diancie Earth Power vs. 248 HP / 8 SpD Tyranitar in Sand: 152-180 (37.7 - 44.6%) -- 3.9% chance to 2HKO after Stealth Rock
4 Atk Diancie Diamond Storm vs. 248 HP / 252+ Def Tyranitar: 109-130 (27 - 32.2%) -- 72.4% chance to 3HKO after Stealth Rock
252 SpA Diancie Moonblast vs. 248 HP / 8 SpD Tyranitar in Sand: 240-284 (59.5 - 70.4%) -- guaranteed 2HKO
252 SpA Diancie Hidden Power Fire vs. 248 HP / 8 SpD Tyranitar in Sand: 25-30 (6.2 - 7.4%) -- possibly the worst move ever

4 Atk Diancie Diamond Storm vs. 0 HP / 4 Def Excadrill: 61-72 (16.8 - 19.9%) -- possible 5HKO after Stealth Rock
252 SpA Diancie Moonblast vs. 0 HP / 0 SpD Excadrill: 129-152 (35.7 - 42.1%) -- guaranteed 3HKO after Stealth Rock
252 SpA Diancie Hidden Power Fire vs. 0 HP / 0 SpD Excadrill: 218-258 (60.3 - 71.4%) -- guaranteed 2HKO

Take your pick:
252+ Atk Excadrill Earthquake vs. 0 HP / 0 Def Diancie: 372-438 (154.3 - 181.7%) -- guaranteed OHKO
252+ Atk Excadrill Iron Head vs. 0 HP / 0 Def Diancie: 592-700 (245.6 - 290.4%) -- guaranteed OHKO

edit: changed counter to check

 

[silver97]

Counter

Excadrill @ Air Balloon
Ability: Sand Rush
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Earthquake
- Iron Head
- Rock Slide
- Rapid Spin / Swords Dance

Air Balloon Sand Rush Excadrill is an unexpected counter to Mega Diancie. The caveat is that it requires sand from Ttar, so I'll include some calcs on that for completion's sake. Ideally, Mega Diancie would try Earth Power or Moonblast to try to take down Ttar, which is not a problem when switching in Exca. In sand, he outspeeds and OHKOs Mega Diancie easily with either EQ or Iron Head, while Diancie is tempted to stay in to KO him with Earth Power. Sand offense is pretty well known for being able to break through many types of teams in general, and even Mega Diancie does not stop it. Switching into a predicted HP Fire is a bummer, though, and could prevent Exca from ripping through the rest of the opposing team. On another note, Assault Vest Excadrill makes for another counter, although it is outsped and nearly 2HKOed by Earth Power (40.4 - 48%), so the Sand Rush set is preferred.

calcs

252 SpA Diancie Earth Power vs. 248 HP / 8 SpD Tyranitar in Sand: 152-180 (37.7 - 44.6%) -- 3.9% chance to 2HKO after Stealth Rock
4 Atk Diancie Diamond Storm vs. 248 HP / 252+ Def Tyranitar: 109-130 (27 - 32.2%) -- 72.4% chance to 3HKO after Stealth Rock
252 SpA Diancie Moonblast vs. 248 HP / 8 SpD Tyranitar in Sand: 240-284 (59.5 - 70.4%) -- guaranteed 2HKO
252 SpA Diancie Hidden Power Fire vs. 248 HP / 8 SpD Tyranitar in Sand: 25-30 (6.2 - 7.4%) -- possibly the worst move ever

4 Atk Diancie Diamond Storm vs. 0 HP / 4 Def Excadrill: 61-72 (16.8 - 19.9%) -- possible 5HKO after Stealth Rock
252 SpA Diancie Moonblast vs. 0 HP / 0 SpD Excadrill: 129-152 (35.7 - 42.1%) -- guaranteed 3HKO after Stealth Rock
252 SpA Diancie Hidden Power Fire vs. 0 HP / 0 SpD Excadrill: 218-258 (60.3 - 71.4%) -- guaranteed 2HKO

Take your pick:
252+ Atk Excadrill Earthquake vs. 0 HP / 0 Def Diancie: 372-438 (154.3 - 181.7%) -- guaranteed OHKO
252+ Atk Excadrill Iron Head vs. 0 HP / 0 Def Diancie: 592-700 (245.6 - 290.4%) -- guaranteed OHKO

I would't call this a counter because it doesn't fit the counter definition:
"Pokémon A counters Pokémon B if Pokémon A can manually switch into Pokémon B and still win every time, even under the worst case scenario, without factoring in hax."
Outside of sand excadrill is 2HKOed by any combination of a move + earth power, with sand excadrill still needs to be careful because hp fire and earth power severely dent it in the switch-in.
However still a solid check

 

[Behexenhaus]

I would't call this a counter because it doesn't fit the counter definition:
"Pokémon A counters Pokémon B if Pokémon A can manually switch into Pokémon B and still win every time, even under the worst case scenario, without factoring in hax."
Outside of sand excadrill is 2HKOed by any combination of a move + earth power, with sand excadrill still needs to be careful because hp fire and earth power severely dent it in the switch-in.
However still a solid check

I would argue that it's still a counter. Outside of sand, Exca has zero chance of getting in anything, so you would never send him in under those circumstances. In sand, he can come in on any move and guarantee OHKO, which is what a counter is. Maybe I'm misunderstanding the definition? Also, Earth Power would not hit Air Balloon Exca on the switch.

 

[firehusky]

I would argue that it's not a counter. It's more of a check, because it takes heavy damage from HP fire, and under a worst case scenario, sand would not be up, however scarf could make a decent check. This is more of a check because it can only switch in under certain cases, like when sand is up. If sand is not up, or if it's balloon is popped, then excadrill is dead.

 

[Behexenhaus]

I would argue that it's not a counter. It's more of a check, because it takes heavy damage from HP fire, and under a worst case scenario, sand would not be up, however scarf could make a decent check. This is more of a check because it can only switch in under certain cases, like when sand is up. If sand is not up, or if it's balloon is popped, then excadrill is dead.

Scarf would risk getting severely hurt by 2/4 of Mega Diancie's moves, while Air Balloon only risks getting hurt by HP Fire. Changing it to check, though, because I suppose having no weather on the field / having Air Balloon popped can count as a "worst case scenario".

 

[Random Passerby]

Check:

Venusaur @ Venusaurite
Ability: Chlorophyll
EVs: 232 HP / 252 SpA / 4 SpD / 20 Spe
Modest Nature
- Giga Drain
- Sludge Bomb
- Hidden Power [Fire]
- Synthesis

Mega Venusaur can easily come in on any move Diancie throws at it (if it's mega evolved) and kill it with a Giga Drain.
The problem is, that if you're not mega evolved, Diamond Storm has a chance to 2HKO you => just a Check.

Relevant Calcs

4 Atk Diancie Diamond Storm vs. 232 HP / 0 Def Venusaur: 190-225 (52.9 - 62.6%) -- guaranteed 2HKO
252 SpA Diancie Earth Power vs. 232 HP / 4 SpD Venusaur: 114-135 (31.7 - 37.6%) -- 90.7% chance to 3HKO
252 SpA Diancie Moonblast vs. 232 HP / 4 SpD Venusaur: 90-107 (25 - 29.8%) -- guaranteed 4HKO
252 SpA Diancie Hidden Power Fire vs. 232 HP / 4 SpD Venusaur: 154-182 (42.8 - 50.6%) -- 2.3% chance to 2HKO

252 SpA Diancie Earth Power vs. 232 HP / 4 SpD Mega Venusaur: 98-116 (27.2 - 32.3%) -- guaranteed 4HKO
4 Atk Diancie Diamond Storm vs. 232 HP / 0 Def Mega Venusaur: 136-162 (37.8 - 45.1%) -- guaranteed 3HKO
252 SpA Diancie Moonblast vs. 232 HP / 4 SpD Mega Venusaur: 77-91 (21.4 - 25.3%) -- 0.2% chance to 4HKO
252 SpA Diancie Hidden Power Fire vs. 232 HP / 4 SpD Thick Fat Mega Venusaur: 68-80 (18.9 - 22.2%) -- possible 5HKO

252+ SpA Venusaur Giga Drain vs. 0 HP / 0- SpD Diancie: 230-272 (95.4 - 112.8%) -- 68.8% chance to OHKO

This EV spread : 232 HP / 28 Def / 244 SpA / 4 SpD
allows you to avoid the 2HKO from Diamond Storm after Stealth Rock while still have enough power to OHKO back with Giga Drain

 

[firehusky]

A pokemon is always a check as long as it can come in on one certain move, and while I agree scarf has a better chance of getting hurt, but keeping your air balloon intact is also not easy.

 

[FullLifeGames]

This EV spread : 232 HP / 28 Def / 244 SpA / 4 SpD
allows you to avoid the 2HKO from Diamond Storm after Stealth Rock while still have enough power to OHKO back with Giga Drain

Thx, I updated it!

 

[Teeny Victory]

COUNTER


Celebi @ Leftovers
Ability: Natural Cure
EVs: 252 HP / 16 Def / 204 SpD / 36 Spe
Calm Nature
- Giga Drain / Leaf Storm
- Recover
- Baton Pass
- Nasty Plot / Heal Bell / Thunder Wave / Perish Song / General Filler

Celebi Quite easily comes in on any of Diancie's moves, avoids the 2HKO and can 2HKO back, while being able to heal easily to come in again and again.
Giga Drain is the preferred option in the first slot, as it easily 2HKOs, but leaf storm can be chosen if you need the Guaranteed OHKO or want more power or something (it should NEVER be used with nasty plot due to special attack drop), but is generally inferior due to Giga Drains Secondary healing effect and lack of Special attack drop.
Recover is for reliable recovery and Baton Pass is for momentum and escaping pursuit.
The Last slot is mostly filler, Nasty Plot can be used to make a sweet boosts passer, heal bell for clerical duties, T-Wave for stopping offensive threats, and perish Song for last ditch attempts at stopping a sweeper, note that Nasty plot allows it to beat CM Mega-Bro, so it is generally preferred.
The EVs are for outspeeding Jolly T-Tar and escape pursuit, enough Defense to avoid the 2HKO from diamond storm after SR, with the rest in Hp and SpD respectively to take advantage of Celebi's good bulk and typing.

Some Calcs:
4 Atk Diancie Diamond Storm vs. 252 HP / 16 Def Celebi: 160-189 (39.6 - 46.7%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
252 SpA Diancie Moonblast vs. 252 HP / 204+ SpD Celebi: 136-162 (33.6 - 40%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
252 SpA Diancie Hidden Power Fire vs. 252 HP / 204+ SpD Celebi: 116-138 (28.7 - 34.1%) -- 1.9% chance to 3HKO after Stealth Rock and Leftovers recovery
0 SpA Celebi Giga Drain vs. 0 HP / 0- SpD Diancie: 168-198 (69.7 - 82.1%) -- guaranteed 2HKO
0 SpA Celebi Leaf Storm vs. 0 HP / 0- SpD Diancie: 288-342 (119.5 - 141.9%) -- guaranteed OHKO

Edit
sorry for stealing it ArchPhantom , just take it next time, I had plenty more mons in mind :)

 

[Puralux]

Reserving Registeel

 

[Random Passerby]

Name : Aggron
Status: Check

Aggron @ Aggronite
Ability: Sturdy
EVs: 252 HP / 16 Def / 240 SpD
Careful Nature
- Iron Head
- Ice Punch
- Earthquake
- Stealth Rock

Mega Aggron can come in on any attack not named Earth Power, tank another attack and OHKO with STAB Iron Head. Ice Punch allows Mega Aggron to check dangerous threats in OU such as Salamence, Dragonite, Garchomp, Gliscor and Landorus. Earthquake allows Mega Aggron to hit Pokemon that usually threaten it such as Heatran and Excadrill. Stealth Rock is there if you predict a switch but if you predict wrongly, it can be bounced back and Mega Aggron will likely be KOed by the 3rd hit.

(Note: Stats readjusted for Diancie-M)

Calculations:
252 SpA Diancie Earth Power vs. 252 HP / 240+ SpD Filter Mega Aggron: 144-171 (41.8 - 49.7%) -- 24.2% chance to 2HKO after Stealth Rock
252 SpA Diancie Hidden Power Fire vs. 252 HP / 240+ SpD Filter Mega Aggron: 97-115 (28.1 - 33.4%) -- 31% chance to 3HKO after Stealth Rock
252 SpA Diancie Moonblast vs. 252 HP / 240+ SpD Mega Aggron: 76-90 (22 - 26.1%) -- 82.4% chance to 4HKO after Stealth Rock
4 Atk Diancie Diamond Storm vs. 252 HP / 16 Def Mega Aggron: 38-45 (11 - 13%) -- possible 8HKO after Stealth Rock

0 Atk Mega Aggron Iron Head vs. 0 HP / 0 Def Diancie: 424-504 (175.9 - 209.1%) -- guaranteed OHKO