The big ademantine golem guarding the stairs? You can only damage him with
Acid damage as far as I know. If you area wizard or a sorcerer, use
Mestil's Acid Sheath. If you are a fighter, get a weapon with
Acid damage.
Highlight the text to see the spoiler words.
Hope that helps
Edit: Or is he the one who runs around and ressurects the other golems?