I use Calm Rage when fighting bosses, but then I don't fight bosses till after level 30 or 40. And I play only as a Mage. In KBAP that is, in KBTL the mage was only class I didn't used (though I didn't like paladin) so probably I'm trying to make up for this now
Why I use some of the spells: Magic Pole-Axe is physical damage, that's whay it can be handy sometimes. Avenging Angel is many directed ligthtnings. Phoenix is the best summon, better than Infernal Dragon (I found that spell, and am underwhelmed). I use Oily Mist with Phoenix a lot (I cast it in place where phoenix will land a moment later), it makes demons susceptible to fire, which is ironic. Flame Arrow is cheap and at higher INT, levels, always burns, unlike Fireball or Fire Rain. Fear disables levels 1, 2 and 3 when fighting without witch hunters or inquisitors, and so on. Every spell has some use, but some spells are more useful. I don't invest in spells again undead, because undead enemies are usually weak, and Healing hits them nice, for instance.