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Voted for Earth and Air.
Earth is 100% a MUST-HAVE.
City Portal is the queen of all spells; explosion, slow, restoration – I can't imagine playing without these spells.
Air is definitely amazing with its speed boost, but the best thing about it is the Dimensional Door.
If I have City Portal and Dimensional Door with an Earth and Air expert, the game is over. You can move around the map with incredible speed.
The problem is that Dimensional Door doesn't appear very often, though.
That's why Air is secondary.
In general, Water and Fire are also very strong schools of magic, but it depends on personal preference.
I would love to level up all schools, but a hero with a strong magical focus isn't always good. If you encounter an opponent with Red Ball + Attack + Defense + Ranged Attack + Leadership + Tactics... it's game over.