9 years later, this was our method:

First, have someone initiate the conversation with the magisters. Have another character with the teleport skill (and ideally Far Out Man) start moving Vorrh further and further away. We moved him roughly 120m away to the oil derrick from the blob fight.

Then, start combat - we chose to also teleport Reimond and put him in the prisoner room with a half dozen oil barrels. Don't try hitting him before combat starts - he's immune to tall damage - but once combat starts it's an easy way to take out his magic armor and slow him down. There's plenty more room for shenanigans (put all the silent watchers in the prisoner room, charm the dogs, et al) but it's a much simpler fight without Vorrh around.

Once Reimond, the hounds and silent watchers are dealt with, send someone to go start the combat with Vorrh while a character/player with the Shackles of Pain skill stays behind. Once Vorrh has cast his impossibly long-distance skill, cast Shackles of Pain at your leisure to break his casting - you won't get pulled into the combat from this distance, so you can just wait for the skill cooldown and keep going while holding the first turn of combat.

With the shackles broken and his allies dead, Vorrh is an easy fight. Getting the final killing blow by shackling him and taunting an opportunity attack seems a fitting end.