Well i had the same problems with this demon guy in Bota. There is a kind of easy way to burn him down.

You walk down to Thorngrin (http://www.wowhead.com/?npc=17978) with your tank and just park your toons on the right upper ledge. cast Totems and burn him down.
if you'll loose aggro with your tank, he has to walk all the way "upstairs" and towards the ledge to reach your toons - so enough time to rebuild aggro on your tank!

Hellfire is real pain but standing on this ledge won't reach you toons - you can just burn him down ( should stop damage cause of aggro reset). if one of your chars
gets sacrificed you should move him towards thorngrin.

thats easy as hell =)