Did this fight on a solo run, Knight with some Necromancy. I had tons of initiative so I always move first. Drew aggro from Alexandar and his goons, killed the gheist and the rogue and then teleported Alexandar back into the middle of the keep when the worm spawned and stayed far away enough so that they whittled eachother down and the worm killed the mage, then I phoenix-dove into the fray again and started hacking at the worm with all I had. It was still a close call, as I forgot to bless myself and the worms attack whittles down magic armor, not physical armor, which I had very little of. If I hadn't had a few health potions I don't think I would've made it.
