Question

How does ash’s greninja turn into ash-greninja without defeating an opponent in it’s regular forme at first?

Comments

So, are you asking if it is possible or how to do it? :P

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He is talking about ash in the anime’s greninja

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The games and the TV show are made by completely different people. There are probably more differences than similarities.

Answer 1

The anime does not follow the logic of the games.
Source: watching the anime

Answer 2

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.