Here's another trick for the Changeling in the Boss Hive. Keep a mid level goo blob around. When you want to kill a higher level monster, alternate attacks on it and the goo blob. You'll have the 75% physical resist for each hit on the higher level monster and take less damage overall. Don't kill the goo blob so you can keep it around to do again. Call it Goo-pong.
Of course it doesn't work against anything with a magical attack, and will depend on the identity of the higher level monster in other ways. But this is a hilarious way to take down Gharbad in particular and also some other slugging-type physical bosses like Frank or the meat man.