There's a lot of "depends" in any answer we could give you.
Like, depends on what you're killing (a lot of hit points, slayer vulnerability, etc?)
Depends on your armor (a lot of SDI? a lot of mana regeneration?)
For example, I have a mage (wrestle/resist/spellweaving) who has 62% SDI and a lot of mana regeneration. With a spider slayer spellbook, he can kill Navery Nighteyes fastest with just fireballs. Flamestrikes costs too much mana forcing me to meditate a lot, but I can literally fireball all day long.
On the other hand, my mystic (no magery) has only 15% SDI and can kill smaller things (demons, for example) with a spell plague and nether-bolt spam. For bigger things, the rising colossus is the killer-of-choice.