Dumb luck

The obsidiman and the troll were rendered all but useless in the first few rounds, although thankfully Slate was able to get a few hits in and cause some significant damage. The deadfall trap was completely ineffective.
I managed to hit it for nothing but did manage to knock it down with an Icy Surface spell.
Morgue's gadfly spell might have been helpful but the effect wasn't immediately evident.
The true hero of the day was definitely Arumvandar, our Archer. His damage might not be obscene like Slate's, but his ability to consistently hit is incredibly valuable.
So yes, we defeated the horror, but it was getting really close. If it could have gotten two or three more successful attacks it would have quickly crippled us.