I know I dealt with the slith horror after obtaining dispel barrier at solbergs tower. How long after I can't remember.But it seems I did it early compared to some struggling members in here.
So I can add to strategies.
I don't use a real specialized archer. I have two melee fighters all human, one pole, one sword.
I steel skinned, augmented, and hasted the whole party.
then starting the fight just outside where the horror don't see you, and charging with a fighter that leg sweeps, next fighter going in for braking shield.
now my priest going in close enough to be able to remove charms and centered so everyone will benefit from mass healing. I think smite worked too, but I don't remember now.
my mage getting in, now and trying to slow the horror. As soon as the horror is slowed I'd summon help w wizard, and depending on how great help I'd get, I might even haste the aid.
otherwise I moved back the wiz a bit and used lightning spray for damage, since this also worked well on the shades the horror summons it's great.
Whenever some character was death cursed I'd generally move them to a full basin, in the order of them receiving the curse.
For some reason, my fighters were tough enough to live through the serious damage spell the horror lands on them, if healed in between.
I really noticed that the summoned creatures did quite good damage on the horror. And when killed I liked to get them up asap again. If you get a worm, or a fire lizard protect it. they do great damage especially hasted.
another thing was to keep the momentum. keep slowing the horror and keep up pressure, leg sweeping (my fighters didn't have more battle skills than to this level).
the first time I did this I missed that one of my fighters had a death curse and after the battle he died. So I had to do it again since I wanted him to receive the reward I got behind the barrier at the priests place.
three missed tries, and then I did it again. getting a bad start, my fighter not hitting when leg sweeping / shield breaking, my wiz not succeeding slowing, was a problem..
but the reward is awesome when you win this one