Well arena is pretty easy that early in game because everyone else won't be having ninjas also (arena is local server only for the first 2 months)

I'd recommend using main ninjas in position 1 for start always, but for now you can't make a real good team, at least focus on getting that sasuke/naruto/sakura to 3 stars.

Also for arena you need to learn game combat mechanics and enemy ninjas it will increase your chance of winning.