Level 13-14 is when you get access to level 5 spells.
1) Slow + Phantom (+Hypnotize?)
2) Necromancer... and there's nothing more to say (it's too easy for him), he's been strong from the beginning... Slow + Poison, and he walks around the map while you're fighting (knock-knock, knock-knock, who's there? It's Death, and what? And THAT'S IT!!!)
3) Nightmares + Implosion, and that's it...
4) Druid, and everything is clear, summoning dragons and phoenixes.