ToME: the Tales of Maj'Eyal

How does one kill the Glass golem boss in the "The Illusory Castle"?
Despite being in the throne room, he just teleports back to heal. I guess I should somehow use these glass walls to stop his teleport, but it doesn't matter where he stands, he always just ports back.

Yeah, sometimes instead of teleporting he runs into the glass walls. I'm not quite sure how to do it consistently either, but it is possible.