1. Summon infernal pet, make him passive.
2. Stand near Karem, let him hit you and spam pet attack at same time. Karem will attack only infernal from now on.
3. You can move pet (don't forget to tell him to attack Karem again) to bring Karem close to Raest.
4. Stand at max range of your Mortal Coil, dps Karem, but save cooldowns for Raest.
5. Most important. You must time Mortal Coil perfectly. As soon as Raest will start talking (at 33% Karem), hit Karem with Mortal Coil.
6. Karem won't go for Purgatory (he still fight with infernal), Raest become target dummy, you only have to be careful with basic adds. I don't know if he ever hits another phase, at 930 item level I burned him rather quickly.