Hey Henk, harder then it seems huh?
Anyways, you have two niches (left and right of the demon portal)
Make sure you put the pitlord on passive and guide him there right after the cataclysm, right after soulshatter so Kan will move to your pitlord. When the felhunters come out of the portal ROF them, pop cd's conflagrate the first, then bane, then chaosbolt the second followed by a rof, two felflames and a couple of shadowburns... Whatever happens make sure you finish your cast of chaosbolt on the third felhunter. Then the fight continues....