Explosion
The normal Fireball might be a good weapon against weaker foes, but the stronger ones laugh at it. But as your foes become stronger, there also comes available the more potent attack spells, like this one. It sends out a normal fireball, but on contact it explodes with the force of a powder keg, causing massive destruction!
This spell is not only good for killing tougher foes, but also to blow up massive doors and other obstacles.