I found that puppies were eating my enslave when karethad was aggroed to me. I'm not sure if this was a bug or not but in the end what I did was to pop soul shatter AS he starts to summon the pups. make sure you are at the opposite side of the portal to your PL, RoF the portal havoc chaos bolt the first two (this should leave them both under 20%) then shadow burn them both, leaving you with at least 3 embers for the third doggy. You should be back pedalling the whole time so as to avoid turning the doggys.
The real key to this fight is maintaining aggro on your pitlord. The best way to do this is to make sure you are using your gateway on cooldown and using PL breath on kanrethad on cooldown. I had breath macroed so that it would target me. This way you can cleanse debuffs from yourself and hit kanrethad too (by standing with kan between you and your pitlord). By properly managing aggro you should get almost NO chaos bolts and any you do should be dealt with by popping sacrificial pact, twilight ward and unending resolve (you will take 0 damage). Make sure you are at 100% hp when popping sac pac or your shield will suck. You have a SICK heal (demonic siphon?) from the pitlord so you should never be below 100% health as chaos bolt comes in.
For imps just RoF the portal, F+B immolate and curse of elements, shadowfury stun the imps then position yourself right in front of them and your pitlord just behind you. Then use your fel flame breath and they should melt.