I know this has a BA, but I want to point just a little bit out.
So, Greninja has an ability called Battle Bond, and if you faint another Pokémon you can transform into Ash-Greninja. The ability else in the anime because it's hard to express in a game while you are fighting in "full potential". So the games decided to make it easier and said if you faint a Pokémon you are fighting in full potential. And, you can't stand in there for a while and wall all the attacks since Greninja's defences aren't the best. So you'd have to be in form to OHKO or 2HKO or 3HKO (dunno) a Pokémon.