I will try to do something with Cloyster.
Defensive Cloyster anyone? Its massive defence stat and huge armoured shell suggest to keep it a bulky arse that doesn't die, in a similar vein to Ferrothorn.
Instead of taking obvious Shell Smash + Skill Link routes or making this thing even more Physically defensive than it already is, which would be heading into Aggron territories, I opted to make this a bulky wall that can take on even the most powerful mixed wallbreakers.
I still added a little on physical defence to compensate for the loss of Leftovers on any defensive set.
I am aiming to stop Talonflame and Aegislash, while checking Charizard and friends. It should be capable of eating Flare Blitzes, Brave Birds, Head Smashes and Sacred Swords, while being a Water-type, which puts a large strain on it.
Typing and Ability:
At first, I wanted Filter, but I considered that a little obvious and it would make it outclass Empoleon too.
I also considered Rough Skin.
However, it didn't solve anything. I wanted this thing to counter Talonflame and Scizor as well as to check other Fire-types, and Aegislash to an extent, and for that, punishing U-Turn by making it a resisted move and quad-resisting Bullet Punch would be vital. Forcing it into Superpower would be awesome.
Water/Ghost would render it vulnerable to the Pursuits, as well as Shadow Balls from Aegislash.
While Steels are common in OU, Water/Steel is rare (only Empoleon has it), and it still doesn't really outclass Empoleon because Empoleon has Defog as well as Roar and Stealth Rock while being capable of carrying Leftovers. It also eats your Mega slot which you may or may not want. However, Water/Steel is weak to Sacred Sword, and still neutral to Flare Blitz.
The Steel-typing also reduces the damage it takes from Stealth Rocks, which is invaluable for a defensive Pokémon.
There are other typings, like Water/Rock, but Water/Rocks are plentiful, and I believe more suitable for Smashing as this typing is better offensively than defensively. Superpower and Sacred Sword would remain issues as well, and Iron Head, Head Smash, U-Turn and Bullet Punch would be no longer be resisted, and Solar Beams and Energy Balls from the Fire-types it is supposed to counter would hurt.
Iron Barbs would punish U-Turn and Bullet Punch with this new typing, but it still took too much from Flare Blitz and Fire Blasts to my liking, but combining it with Heatproof should do the trick.
Aegislash can now still 2HKO it after SR + Spikes with Sacred Sword, but Cloyster-Mega is still a counter without the Spikes, as it keeps outspeeding Aegislash quite handily and punish the physical moves with a combination of its ability and Spiky Shield. The special moves from Aegislash are either heavily resisted or hitting neutrally, and Aegislash cannot boost those moves. Furthermore, if Aegislash attempts to set up further or uses King's Shield, it risks eating a Draining Bubble, which will be very annoying for Aegislash to deal with.
Life Orbed Swords Sance boosted Sacred Swords still OHKO, but Aegislash does have to eat a Scald or Hydro Pump prior to that.
There is also the double-MEVO Charizard, and I believe Cloyster should be capable of standing up to both. It however, does not beat them every time, due to Focus Blast and Earthquake, respectively. Focus Blast, however, needs to hit twice in a row, which only has a 49% chance to occur.
Moves: I did not want to give it Recover, but as a defensive Pokémon without leftovers, I believe it needs at least some way to recover health. I also wanted it to force switches to abuse the hazards it sets, in a similar vein to Ferrothorn, so I gave it a Leech Seed clone in order to do so.
However, said Leech Seed clone competes for a moveslot with Freeze Dry.
Stats:
Def and SDef are increased for obvious reasons, SpA a slight bit to 2HKO Aegislash-Blade after Stealth Rock with uninvested Scalds.
The result:
Mega Cloyster (Cloysterite)
Water/Ice --- > Water/Steel
Abilities: Shell Armor/Skill Link/Overcoat -> Heat Armor (Heatproof + Rough Skin combination)
BST: 50/95/180/85/45/70 -> 50/95/195/100/110/75 (0 / 0 / +15 / +15 / +65 / +5)
New moves: Spiky Shield, Freeze Dry, Draining Bubbles (Water-type Leech Seed clone but with a twist; Steel-types are drained for 1/4th of their health every turn instead of 1/8th.
However, all Water- and Grass types are immune to it and heal 25% of their health instead when hit by this move while it has no further effect, even if they have a secondary Steel-typing (like Ferrothorn, Empoleon and fellow Mega-Cloysters))
Optional moveset:
Cloyster @ Cloysterite
Ability: Shell Armor (Heat Armor after mega evolution)
EVs: 252 HP / 24 Def / 232 SDef
Bold Nature
- Spikes
- Spiky Shield
- Scald
- Draining Bubbles / Toxic Spikes
A physically defensive spread is also an option. Its base SDef is increased enough to make sure it doesn't die to random special attacks anyway.
This spread, however, ensures Cloyster-Mega can take two Sacred Swords after Stealth Rock from Aegislash, while also avoiding the 2HKO from Shadow Ball on its lower SDef.
Talonflame can't do jack to it, and even if Cloyster has to eat an U-Turn, Talonflame will pay the price in Heat Armor damage, losing 1/8th of its health.
Even Overheat is not a problem with its newly buffed SDef and Heatproof to tag along it.
Scizor's normal moves won't do much to it, and even Superpower only 3HKO's. Add onto that that Cloyster outspeeds, rendering it helpless.
It does not counter or check Rotom-W, but it can stay in on a Volt Switch from the defensive variant, something that would have brought Cloyster down to its Sash without the buffs.
It also counters both formes of Charizard moderately well, resisting the STAB combinations, but it needs to be careful of offensive Dragon Dance sets that carry Earthquake. Bulky Dragon Dance is still countered, more or less.
Calculations said:
252+ Atk Aegislash-Blade Sacred Sword vs. 252 HP / 24+ Def Cloyster: 120-142 (39.4 - 46.7%) -- guaranteed 3HKO after Stealth Rock
252+ SpA Aegislash-Blade Shadow Ball vs. 252 HP / 232 SpD Cloyster: 120-142 (39.4 - 46.7%) -- guaranteed 3HKO after Stealth Rock
Life Orb sets do 2HKO after Stealth Rock, but Aegislash rarely runs Life Orb as it reduces the effect of its massive bulk. Aegislash can switch into Shield Forme to avoid the 2HKO from Cloyster, but Draining Bubbles followed by a Spiky Shield would throw a wrench in those plans.
252+ Atk Choice Band Talonflame Flare Blitz/Brave Bird vs. 252 HP / 24+ Def Heatproof Cloyster: 58-70 (19 - 23%) -- guaranteed 5HKO after Stealth Rock
252+ Atk Scizor Superpower vs. 252 HP / 24+ Def Cloyster: 144-170 (47.3 - 55.9%) -- guaranteed 2HKO after Stealth Rock (NOTE: Will fail to actually 2HKO due to the Attack drop from Superpower. It will, however, OHKO with a 70% chance if Cloyster switches in on the Swords Dance. The remaining 30% is the chance that a Scald burn occurs).
+2 252+ Atk Scizor Superpower vs. 252 HP / 24+ Def Cloyster: 286-338 (94 - 111.1%) -- guaranteed OHKO after Stealth Rock
4 SpA Rotom-W Volt Switch vs. 252 HP / 232 SpD Cloyster: 120-144 (39.4 - 47.3%) -- 0.4% chance to 2HKO after Stealth Rock
252 SpA Mega Charizard Y Fire Blast vs. 252 HP / 232 SpD Heatproof Cloyster in Sun: 118-141 (38.8 - 46.3%) -- guaranteed 3HKO after Stealth Rock
252 SpA Mega Charizard Y Focus Blast vs. 252 HP / 232 SpD Cloyster: 228-270 (75 - 88.8%) -- guaranteed 2HKO after Stealth Rock
+1 0 Atk Mega Charizard X Earthquake vs. 252 HP / 24+ Def Cloyster: 136-160 (44.7 - 52.6%) -- 80.1% chance to 2HKO after Stealth Rock
0 SpA Cloyster Scald vs. 252 HP / 0 SpD Mega Charizard X: 99-117 (27.5 - 32.5%) -- guaranteed 3HKO after Stealth Rock
However, Mega Cloyster still needs to be extremely wary of powerful Super-effective moves; Conkeldurr, Alakazam and the likes all have ways to 2HKO it with ease.
Also, Cloyster needs to Mega-Evolve first before it is actually capable of countering or even checking these things, which can be a bit of a hassle.
Last, it can also still run a Shell Smash set, but only on the Special side, considering Skill Link is gone. Both its Speed and Special Attack are increased slightly, but... its main selling point, its bulk, is heavily compromised.