Some initial strat stuff I thought up:

• Arazen and Fawbob will have to be the main magic force. They should be moved to the bottom corners of the map where they can stand safely.
• Shuriken, Fooruman and the summons will have to be the melee force used to attack Abydos.
• The summons need to strike first for that they leave no body behind.
• Shuriken and Fooruman are required to be the melee force, for they have Feat and Custard's Last Stand respectively. (Risp told me Rad has a counterstrategy against those attacks though)
• If Shuriken dies in front of Abydos, use Animate Dead (NOT RAISE) to move her out of the way.
• If Fooruman dies, raise him.
• Concentrate on using the attack spells to wipe out the Abyss Monsters. Slay, Lightning, Disbalance, two Foshizzles. Will be enough.

And keep in mind that while Abydos is anti-magic, he can be raised, and I take a bet that the Abyss Monsters do have Raise. That also means that he could be Disbalanced using Feat (I think?), which would be great.