Alternatively, you can trick the Guardian into walking straight into the lava that will appear at the top of the stairs if you're close enough.
You'll need a character that can sneak past the Guardian (I used the invisibility spell). Give him a teleporter stone. Once he's in a safe place at the top of the stairs, teleport the rest of your party to him. Then use the teleporter stones to get past the lava.
By this time, the Guardian should be aware of your presence and start attacking you. He'll shamble up the stairs, wade into the lava and that will be it.