I think Ghost Sword and Fireball might be two best direct-damage spells.
You guys have lots of cool spells I'm not finding in my game! But the best one that I use all the time so far is "Fit of Energy," because it gives me two turns with one unit. I always give the extra turn to my best ranged unit, so usually on my first turn I can eliminate any single stack of enemy troop that I choose to.
I have about 170 Bowmen, 20 Cannoneers, 30 Alchemists, and 40 Evil Beholders. The bowmen do the most damage if the enemy has low defense, otherwise the Evil Beholders are best because they do magic damage instead of physical.