I did similar tactics, but didn't do much with the spells since I thought the final boss in IWD had close to 100% magic resitance.
I had my stoneskin mage run around and distract the golems while I peppered the boss with Arrows of Piercing.
I did add fire resitance to the fighters before the battle, knowing (after a few reloads) that the demon would cast that fireball/teleport spell. I tried to avoid the traps as best I could, since I think one of the traps dispelled magic.
I played my first game through with a Fighter, a Ranger, a Mage, a Bard, a Druid, and a Thief. When I took this party into HOW I dropped the Bard and added a Paladin (needed another tank). Looking at your party, I'd probably drop the Ranger. Druids are great summoners.
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Most Subtle Official Straightman of the Laughing Hyenas -- Grammarian Brigade.