Hots, shields, PoM.

Pull him to the wall with your tank to give your dps more room to move.

Arcane missles and fireblast rotation.

As for staying away from the flares(assuming your healer is your main), try setting your dps in one spot and your healer about 15 yards away. When a flare is called on your healer, just move easily. When a flare is called on your dps press follow and then set them back to dpsing where they stop while moving your priest to a new location. Try do do this swapping at max range of course. If your tank is called, just strafe him to the side along the wall a bit.

He's meant to be hell for casters, goodluck.