Level 26 Mage on Impossible, and I still use direct damage spells with frightening efficiency.
My Kamikaze spell does a max over over 8000 damage. Fire Rain does just over 6000 damage. Ice Snake does 6750 damage to the initial target.
42 intelligence will do that.
Actually it'll be 47 soon, since I'm about to finish "draining" the Pain Skull.
I actually find Geyser kind of weak for the mana cost, unless it's a battle with exactly 8 stacks which do not resist physical damage (much or at all).
Right now I have a full army, since the Pain Skull halves my Mana. But I actually got from level 18 to level 25 by winning battles against hordes in Death Land and Elinia with a single stack of Necromancers. No other army. That's due to a little Invisibility abuse.
THAT is one of my favourite/most powerful spells in the game.
With the Might tree undead upgrades, your Necromacers have 9 Initiative (8 if enemy has Black Dragons present). It means you get to cast first, before any enemy action. Invisibility is your first spell, then just do Kamikaze on the next three turns. Invisibility lasts 4 turns (and you can replenish it without ever getting movement from the enemy, with the wait command, thanks to high Initiative, the enemy finishes its turn before you have to press "defend"), so the Kamikaze cycle starts before even your first cast of Invisibility expires.
At 5500-8500 damage per each bomb for 15 mana, the Kamikaze cycle is a wonder to behold.
After that my Necromancers raise the dead three times, and these mop up the survivors (if any).
And the wonderful thing is that while Kamikaze can't be cast on Black Dragons, it CAN be cast on the units NEXT to them.
If for some reason I'm running out of mana, there's the magic spring + poison rain trick. Or even just summoning a Demon Gate and then using magic spring on the demon. Doing so in conjunction with an Ice Ball taking away some attention from the summoned stack helps it survive.
So, even on Impossible difficulty, you can still go with direct damage spells IF you push your Intelligence (and mana) up at the expense of everything else. My Attack is at 7 and defense only at 15, IIRC. Really low, yes, but then it doesn't matter when 90% of the damage comes from my spells and 90% of enemy units never get to hit anything.
Of course if I was still playing a Warrior or Paladin on Hard/Impossible, I'd get myself a nice stack of Skeleton Archers and use Dragon Arrows on 'em. Now THAT is some insanely broken damage (10 Black Dragons gone in a single shot from ~1500 archers)!