My [in game] Roserade destroyed him with Sleep Powder, Giga Drain, Shadow Ball, and Petal Dance. I got quite lucky though XD.
I've never gotten much past the first battle with him though, sadly.
I fought his first battle a second time with Articuno [Mind reader + Sheer Cold, Roost, Ice Beam XD], Metagross [Agiligross], Starmie [lead w/ Life Orb]. He went down real quick like... except that Milotic!
But as a rule:
Rhyperior is a wimp
Dragonite will give you a problem if you don't get lucky or don't have something with Ice Beam
Milotic is EVIL