I don't recall anything specific I had to do to deal with the opposition down there, but by the time I did the Planes, I had a pretty tough party. One recommendation is not to use Fire or Air spells, because the Boulders and Chargers are immune.
Do you know the best way to handle the spell Shrapmetal? Your necromancer should have it, and, used properly, it is the most destructive spell in the game. What you do is pick one opponent, rush up to it until you're right in contact range, and then hit Return to enter turn-based mode. Then cast the spell when your necro's turn comes up. If your necro has GM in Dark magic, he/she will cast 9 fragments of metal, each doing a minimum of 10(1d6+6) - an average of 95 - points of damage, and that single opponent will get the benefit of every last one of them. Even though your opponent's resistance will degrade the attack somewhat, there's not much that will stand up to that treatment for long.
If you can chop one opponent down before you have to run and heal, you can clear the position, although having to run and heal repeatedly will take quite some time and your party's armour will be in tatters by the time you've finished.
I hope that helps!