The edges provide some cover. and the devils do the classic 'hug the walls, then come out to take a shot'....So open the door, launch some mass-damage spells, tuck to the sides.....close the door or take the shaft down if necessary.... but you should be able to launch a first strike, hide in the edges of the doorframe/portal while they shuffle to the sides of the room, and take out the ones that step out in front of the doorway to shoot at you.
The biggest annoyanc is the devil lords/masters ability to drain spellpoints. May need to save-and-reload a few times if they squelch all your spellcasters. Actual damage and attacking them is pretty straightforward.
