velen+uther+another holy user (xyrella or anduin).
pass 1st turn.
turn 2 velen's blessing + avenging wrath + another holy skill ( i usually use holy word: salvation for holy skill that dosnt do dmg, but you can use a heal or any other skill with speed <6 - i have velen's blessing at r3 )
you cast 2 time avenging wrath on merc enemies.
it take a bit, but it's the fastest way i find for this task.