My Aruseus would be my Double Battle star.
I'm probably choosing fire...BUT I'm gonna have to teach it Sunny Day, flamethrower, solarbeam and Judgement pebble, then carry my Mew that knows Transform, which will hold the Grass plate. This way, I'll effectively have an Aruseus with two working types.
It would also work if I gave its Double Battle partner a Grass plate and taught it Trick.
Come to think of it, a Water/electric duo would work well, too (until I came across a grass type opponent)