Sneed is a pita because it's basically two bosses. I had a lot of trouble at that level (and still at 20) with him, due to lack of mana and spell resists. However, he does stick around for a while after you kill the Shredder. I'd suggest at the moment that you aim to just kill the shredder, take the wipe and then run back and do Sneed himself. As far as I know neither should despawn.