My strategy for killing Grah-Hoth was simple and extraordinarily effective.
Once the battle started, I immediately opened the secret passageway leading to his private chambers (button behind his throne) and ran my whole party in there, thus funneling him and his minions through a tiny hallway one at a time.
From his chambers you can also blast away through an impenetrable barrier. Haha.
Grah-Hoth is only challenging if you fight him on his terms. Sun-Tzu would be proud of me I'm sure.