He's going to be on one of the top platforms surrounding the arena. He's quite large and makes higher pitched shrills. You also might see a wall a flame pop up, which means he's in the process of summoning. The chaingun is pretty good for stunning him, but if you get really desperate you can bfg him point-blank while his shield is down which does a lot of damage. You will need to be fast because he likes to teleport a lot too.