How do I kill the iron golem in de'Arnise Keep? Any good ideas?
Any special weapon/spells needed?

MELF'S MINUTE METEORS!!!

Then you'll have enough Iron to build the Eiffel Tower. (okay.....a mini version anyway)

Or if you still can't kill him. Buy Azure Edge from Bernard in the Copper Coronet. Not only is it a +# weapon that you can throw that can hurt irom golems, but it also has a chance of outright destroyin undead as well. I killed many vampired this way as they attempted to charge at me. A good shot between the eyes knocked them into oblivion.

Another thing you can do is cast lower magic resist first before casting anything else on them if you choose to kill it with magic.

Use any long weapon(staff/halberd/2-hd sword, etc.), must be +2 or better, get the golem trapped in doorway, stand at extreme melee range and whack away. GOLEMS ONLY HAVE A SHORT ATTACK RADIUS! They can't hit you back! Use someone with a good save against the poison gas cloud. For this reason Jaheira was my Golem killer. Works really nice on Adamantine Golems later - TOUGH.