Watch the colors of the shields the wizard is using and send your minions of color to break them. He'll fall to the ground after each breakthrough and then you can sen all your minions to him and assist them with your powers and weapons. Dodge his attacks and collect stuff from the barrels around the arena - surely you'll find some goodies there. There are spawning pits on the sides too so you shouldn't run out of minions. The bad thing is: there are lots of the crazy ones and they are particularly annoying. The best way of dealing with them is to use your spells of mass destruction.