Start by summoning "mordikans sword(sp)" or an invisible stalker and have it attack slayer....then concentrate on taking out all of the demons (one at a time with everyone in your party attacking one and then moving to the next). They will fall pretty quickly and then you can go after slayer. I did this and my invisible stalker had slayer backed into a corner and had taken 0 damage. It was actually a pretty easy fight and I was a bit disappointed, but hey it worked
Good luck!
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Never try to teach a pig to sing; it wastes your time and it annoys the pig
I've got to admit it's getting better, it's getting better all the time