I have attempted to beat this challenge 3 times and each time it crashes when my druid kills abhorath last. And by crash i mean the game completely freezes in the middle of him being hit for fatal damage. This happens whether its a skull kill or an ability kill, it just freezes then crashes out.
My team is Herdmaster (epic), Valkyrie (epic), Alchemist (epic), Druid (legendary)
EDIT: Upon further investigation this seems to be happening fairly consistently when my druid kills ANYthing in karakoth challenges that is in last position.