Good spells to use against a Horror are summoning spells and personal protection buffs. Anything to make it hit anything but you. It's the bodyguards that will help save a mages flaming bacon. Horrors are nigh indestructible by magic.
I never use horrormarked gear until the lategame when I can rely on the strongest bodyguards available. I usually have one of the worse horrors show up within five turns.
