Use items to boost your damage (clickies of specific element, like inferno, are stronger- the text is incorrect), use lore items.
Use power/wizardry items to get more SP. (No need to get overboard though.)
Use burning hands/acid spray on groups, don't use niacs on rangers/sneaks, if something is left with a sliver of HP left equip the
Eternal Wand of Finger of Fire, and finish it off. In fact, you could as well kill trash stuff with that wand, and save SP for big fights.
Oh yeah, and let the melee types do their job.
Don't use maximize by default. If it's the end fight and you have a bunch of SP left, use it, but if it doesn't end the fight, be ready to turn it off while you still have ~50SP.