I will attempt to calculate that.
...
Alright.
242145133.3% - 284876700%
+6 Special Attack, 31 IV, 252 EV, Special Attack boosting Natured Lvl 100 Mewtwo. Swap Color Change to Mewtwo, hit him with a fire type move to change him into a Fire type, Swap Flash Fire and activate. Use Sunny Day/Groudon.
Lvl. 1 Dry Skin, Special defense lowering Nature, -6 Special Defense, 0 HP and Special Defense IV, 0 EVs Paras. Paras learns Eruption with Mimic. Mewtwo uses Me First nine times holding a Metronome, and Me First causes Mewtwo to use an attacking move all nine times. On the final turn, a Cherrim with Flower Gift uses Helping Hand to boost Mewtwo, as Paras uses Eruption, and Mewtwo uses Me First at full health, and scores a critical hit.
Needs Adaptability, Flower Gift only raises Attack, and Choice Specs tricked on the last turn.
484290266.6% - 569753400%