Yep, that's a tough battle...perhaps tougher than Sarevok himself...
I always summon a load of monsters to surround the demon...then summon here if he starts getting through them and keep my party out of range so they don't get inflicted with a status effect that just leads to death later on.
Emotion (Hopelessness), the 4th level spell is a good one to use...it won't work on him a lot but when it does he'll be at a standstill. If you can find a way to reduce his saving throws before casting the spell it'll be even more effective.
Oh, don't forget to protect your party from fear effects too...the summoned creatures may fall victim to that and some stun/hold effects but the point is that they stand there while you can pepper the demon with magic arrows.
Even with all that it does take patience and luck to defeat Aec'Letec.