+3 weapons or better is right. Also magical fire damage actually heals him (according to PnP D&D).
Tactic I used...
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You can use the ring of golem control to take control of the Iron Golem, and have him destroy the other clay golems. This will weaken him enough for you to handle him with little problem.
Where do you find the ring? I can't exactly remember
I left this area of Nalia's keep (because it was too hard) and came back when everyone had leveled up a bit, so I can't remember if the ring is in the keep proper or not. I think it is.
Another tactic is to kill the clay golems first, then rest up, cast haste on your tanks, and all your defensive spells on your tanks, and have them go at it. Remember you'll need +3 weapons.