I really struggled with this too unlike other boss fights the auto option for this can be more difficult as the AI doesn't always differentiate the correct target I use 2 healers 1 support/sheild drop and 2 attack sorry not sure of there names is the sacred order and dark elves leaders that you pick at the start I suggest you prioritise group attacks and hold off until you can take out a large group vampiric armour helps alot and benediction is a life saver I'm around 30000 Combat efficiency and I'm only clearing level 2 on auto