help with final battle - player/rival/mew vs giovanni/mewtwo?

[bigredwill]

I've got a simple 2v1 battle set up with the player and the rival vs Giovanni and an overpowered Mewtwo. Is there a way I can have Mew sweep in at the last moment for a 3v1 or some kind of thing? I don't want to just start off with Mew in the battle, I want a classic "snatch victory from the jaws of defeat" but I have no idea how to make something happen inside a battle.