Some ideas: raiding, seiging, spell AI and more..

[Cheezeninja]

Regarding Doom Horrors, I just encountered them for the first time in a MP game, playing against Zapmeisters R'yleh using Jotunheim. While they ate everything (including many Jarls) that I threw at them for awhile I was eventually able to make huge headway against them to the tune of massed vampires and counts (free immortal chaff/disintigrate or drain life or skeleton casters) along with Jade amazons and Gyjas fitted with rune smashers, thistle maces, spell foci, flying boots, and AMA's set to mass cast charm. I was able to pay for my own gear with my own stockpile of clams and fetishes, which gave me enough gem capital to completely create the charm strategy in ~3 turns, which was quick enough to allow me to survive. While its quite likely I will still eventually lose, right now i've killed 3 horrors and brought one over to my own side, complete with ring of regen and AMA of his own.

Doom Horrors are far from invincible, if I was going to spend a hundred astrals on something it would probably be wishing for bloodslaves and mass-casting Vampire Counts.