i just beat arceus but i need to know if you also have to beat mewtwo in ex6 in order to catch him in the tower?
You'll have to clear the game, then beat team EX-6, then Arceus should be available in the towers. Hope This Helped! :D
Here is a video on catching Arceus: http://www.youtube.com/watch?v=g390Wx6y_s0&feature=related