When Mew transforms into an enemy in Pokemon Mystery Dungeon will it have the same attacks as the form it transformed into or will it have the same attacks as if it were mew?

When and if Mew transforms it will have the same moves as it did before it transformed. Say Mew had Psychic, Transform, Mega Punch, and Metronome, and it transformed into a Mawile. It would still have the same moves it had before.

It will have the moves it knew if you recruit him. It has its own moves.

When Mew transforms into another Pokemon, it still has the same ability (Synchronize) and moves as it did before. Mew seems to be different from Ditto in that it can keep its own moves. However, the Mew in Buried Relic really only uses Mega Punch, and most of the time it transforms into a Sandshrew.