This took me a while to clue into. You get to the boss, you gonna wanna use your most powerful attack spells. These bosses all have elemental weaknesses, and that's just classic gamer shit. You hit the weakpoint for massive damage.
Don't listen to that voice. Those spells are for crowds. When you have one single powerful enemy, temper is the most broken spell in the game.
It raises somebody's attack, right? But it lasts the entire battle, unless the character gets knocked out and revived. And it stacks with itself. As near as I can tell, it stacks infinitely. Throw in some haste for double apr and you're in business!
In the unmodified game there was a pair of gauntlets you could get for your fighter that would basically just make black mages redundent in boss battles, because you could use it to cast temper on yourself. Jeff Ludwig did something about that in his mod but I forget what.
And I'm obviously being a little facetious with the "Don't listen to that voice" stuff. The game's not terribly difficult and you can get away with a lot of suboptimal play if that's how you wanna do it. It's fun to figure out more effective strategies but you should never feel confined.