[Jazzepi]

You might want to try forging spell foci, eyes of the void, and if possible, rune smashers. These will increase the difficulty of the MR check by the abomination.

Also, I would recommend casting the conjuration spell, whose name I can't recall off the top of my head, that gives every astral mage on the battlefield +1 astral. This should make it easier to spam soul slay.

Jazzepi