Abydos can be targeted with Feat (Rad said so in the first post for this battle).

My thoughts: we take out the Servant as soon as it appears, as it's the most dangerous of the summons (it can heal and make someone invincible). The rest of the summons probably have no abilities, or we'd have most likely seen them by now.

Abydos does have a limited number of spells, and can't actually do that much if we're not standing next to it. We could if we wanted to take out the summons and pit monsters, and just ignore Abydos while the AWA runs round healing everyone (with normal attacks). Get the rest of our summons in, and then attack Abydos. Custard's Last Stand and Ninja Speed would help. We could Animate Dead one of the pit monsters if it leaves a handy corpse, and use that too. Shame we don't have an Unsummon or two.