I use windfury/frostbrand, stand really close to him, positioning myself so that i'd fall into lava when he knocks me back (parachute helps) and pretty much full-on nuke. Adds have low health, never had a problem with those, depending if I'd get into the submerge phase or not (usually depends on whether I get knockbacked all the way into lava or just a bit, most of the time I kill him before submerge though).