When thinking of a single Mon to deal with Mega-Blaziken as your primary team member, I find it preferential to select a Mon that can handle the esteemed trio of Blaziken, Venusaur, and Zapdos near single-handedly. As sometimes Mega Blaziken and Mega Venusaur are both on the same team and which 1 Megas depending on match-up or there might be Life Orb Ken and MegaSaur, I believe all these scenarios are essentially equivalent.
Azumarill might be great for Blaziken alone, but when Venusaur and Zapdos are teammates then things get rough. Same thing for Mega-Altaria, Slowbro, Hippowdon, Feraligatr, and Talonflame. Can they all work? Yes. But for a single Mon, can there be something even better? I believe so.
I present to you guys Mega Latias!
Latias-Mega @ Latiasite
Ability: Levitate
Level: 50
EVs: 252 HP / 36 Def / 12 SpA / 12 SpD / 196 Spe
Timid Nature
IVs: 0 Atk
- Calm Mind
- Psyshock/Psychic
- Draco Meteor
- Roost
Calcs:
Latias-Mega vs Blaziken-Mega
12 SpA Mega Latias Psyshock vs. 4 HP / 0 Def Mega Blaziken: 150-176 (96.1 - 112.8%) -- 75% chance to OHKO
12 SpA Mega Latias Psychic vs. 4 HP / 0 SpD Mega Blaziken: 168-198 (107.6 - 126.9%) -- guaranteed OHKO
Blaziken-Mega vs Latias-Mega
252+ Atk Mega Blaziken Knock Off vs. 252 HP / 36 Def Mega Latias: 78-94 (41.7 - 50.2%) -- 0.4% chance to 2HKO
Zapdos vs Latias-Mega
68 SpA Zapdos Hidden Power Ice vs. 252 HP / 12 SpD Mega Latias: 42-50 (22.4 - 26.7%) -- 15.2% chance to 4HKO
WORST CASE SCENARIO 252+ SpA Choice Specs Zapdos Hidden Power Ice vs. 252 HP / 12 SpD Mega Latias: 78-92 (41.7 - 49.1%) -- guaranteed 3HKO
Venusaur-Mega vs Latias-Mega
252+ SpA Mega Venusaur Sludge Bomb vs. 252 HP / 12 SpD Mega Latias: 57-67 (30.4 - 35.8%) -- 38.1% chance to 3HKO
Latias-Mega vs Zapdos and Venusaur-Mega
12 SpA Mega Latias Psyshock vs. 252 HP / 240+ Def Mega Venusaur: 80-96 (42.7 - 51.3%) -- 2.7% chance to 2HKO
12 SpA Mega Latias Psychic vs. 252 HP / 0 SpD Mega Venusaur: 116-140 (62 - 74.8%) -- guaranteed 2HKO
12 SpA Mega Latias Psychic vs. 252 HP / 0 SpD Zapdos: 76-90 (38.5 - 45.6%) -- guaranteed 3HKO after Leftovers recovery
12 SpA Mega Latias Psyshock vs. 252 HP / 200+ Def Zapdos: 51-61 (25.8 - 30.9%) -- 7.2% chance to 4HKO after Leftovers recovery
As can be seen from these calcs, Mega Latias can tank a Knock Off from Mega Blaziken very reliably and OHKO it a very high percentage of the time in return with Psyshock and all the time with Psychic and that is before it takes a Defense drop from Superpower or any residual damage from hazards/recoil. The spread was built like this because Timid max speed is unnecessary on Latias when Gengar is the most common Pokemon used at that speed tier and is mostly Mega anyway. Also, Mons like Terrakion and Infernape aren't threats to Latias so there's no need for to outspeed them either. The current spread allows Latias to outspeed max speed Garchomp and lets Latias survive 2 Knock Offs from Mega Blaziken a lot more reliably than 0 Def as well as increases the likelihood for a Psyshock to OHKO a 100% hp Mega-Blaziken, which enables Mega-Latias to even Calm Mind up on a Mega-Blaziken if it dares!
60 SpAtt can be used to make Psyshock OHKO over 90% of the time but I feel the greater bulk from the Def investment is better overall. Max SpAtt Latias won't be able to OHKO Mega Venusaur even with Psychic so a bulky spread is better to ensure maximum set-up potential. Also, Zapdos and Venusaur are set up fodder unless RNG comes into play in their favor (Sleep Powder landing and max turn sleep, PRZ and full PRZ when it needs to).
The frustration and despair a Mega Lati, especially this 1, gives a Blaziken/Mega-Blaziken user (such as myself) is well worth the terrible design of this Mega! The sorrow of a trainer who finally decides to run Knock Off to OHKO those pesky Specs Latios, just to find a Mega Latias staring at them with that cute, yet sinister, grin. Steel types may give this Latias trouble, but this Latias covers so many threats single-handedly there shouldn't be any trouble finding partners to handle those pesky Aegislash, Scizor, Ferrothorn, Heatran, Fairies and Choiced Draco droppers! Psyshock has the potential to wreck havoc on Fairies who think they could end this Mon too!
EDIT: "Mega Gengar eats Latis up though." Really? I understand where you may be coming from but let's just leave this as conclusion:
252 SpA Mega Gengar Shadow Ball vs. 252 HP / 12 SpD Mega Latias: 116-140 (62 - 74.8%) -- guaranteed 2HKO
12 SpA Mega Latias Psyshock vs. 0 HP / 4 Def Mega Gengar: 146-174 (108.1 - 128.8%) -- guaranteed OHKO
12 SpA Mega Latias Psychic vs. 0 HP / 0 SpD Mega Gengar: 144-170 (106.6 - 125.9%) -- guaranteed OHKO