Notes:

This can be done by Mega Mewtwo X, which has the highest attack base stat of all Pokemons. Mimic on the V-Create of Victini and using Skill Swap with a Pokemon with either Pure Power or Huge Power. Also increasing the attack stat to the max with the right natures, IV’s and EV’s and Baton Pass on Mewtwo beforehand. For Paras the lowest level possible and 0 IV’s and EV’s in defense, also -6 in defense. Alñ of this is useless though, because the minimum possible of a stat is 5. Also Dry Skin which makes Fire type attacks even more effective and during Sunlight (Harsh Sunlight for the lols). Helping Hand for that extra damage and finally the Fire type STAB which canONLY be done by either Metronome and (luckily) using Reflect type on a fire type Pokemon or giving Mewtwo Protean with Skill Swap and using V-Create to get the fire type. A good ol crit for the extra damage desired. For the maximum damage possible there could be as well and item that boosts fire moves, but given that it already has the Mega Stone and it can’t be given another item it is impossible given that this process is actually legit and can be done with lot of effort. Another possible Overkill is just the last hit of Ice Ball after a Defense Curl given all of this conditions against a x4 weak to Ice Pokemon