A tad late, I know, but it might be of use to anybody else stuck on this particular fight (it is tough, but with the right preparation, it is manageable). Basically, you need to use the Rod of Ruin on the first watcher in order to initiate the fight, which consists of 4 incredibly tough golems and one of each type of elemental watcher steadily spawning elementals.

Before you even begin, make sure your characters are buffed to the max, especially your front-line fighters. It's also a good idea to quickly put a couple of Forcefields in front of either the left or right-hand doors, so you don't immediately become swamped. Then, after the battle commences, concentrate all your attacks on one golem (I usually go for the one on the left, but it doesn't really matter), and when he falls, try to run into the door he came out of. If you're quick, you can get another forcefield up behind you before his pals join the fray, which will give you more than enough time to take the elemental there down, plus leaving you free to use the Rod of Ruin on the elemental watcher there.

From the safety of this alcove, you can now fairly comfortably destroy the remaining 3 golems, leaving only the elementals to worry about. All of them have a weakness, so exploit that, particularly with bombs. In any case, simply move round the room knocking off the watchers in turn, using Forcefield to grant you some peace and quiet to charge up the Rod of Ruin. Nothing worse than taking 10 seconds to charge it only for a lightning elemental to injure your hand and cancel the spell, am I right?