[Taqwus]

Horror marks are indeed cumulative.

It -is- possible to beat horrors (although Kurgi's a bit of a tough customer)... and it should be noted that an astral mage worthy of the Forbidden Light should be able to cast Ritual of Returning as well as the in-battle version. The Solar Brilliance effect may also be useful in certain battles, especially when you don't care much about blindness (ex. R'lyeh mindblasts -- prec 100 attacks hit even if firer is blind).

Magebane would help. It'll boost MR (sort of important for when the Eater of Gods shows up -- given his 'save or die' attack), and when you hit, it'll knock the horror unconscious.