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Initially, I choose warrior heroes because you only cast spells once per turn, but you can attack seven times per turn, which is a plus. Then, it's also important who goes first. Cast Haste 1, and you don't need anything else. Warriors rule, no matter what, because you can easily get by with magic of levels 1-2, without significant mana costs. You can cripple a mage simply by blinding their units; they'll just have to remove the blindness, not to mention various spheres of prohibition or cloaks of defiance. Sometimes, the Explosion spell can turn the tide of battle, but only if two players with equal attack and defense stats meet. If these stats differ significantly, no explosion will save the mage. The most common spells are Slow and Haste.
Based on the post, I'd say: Earth - Slow, Portal (if it's used), I don't include Explosion here because the Expert magic doesn't significantly affect its damage; Resurrection; Air - Haste, Teleport. There's also one useful Expert magic, Water, because of Prayer, and I think Clone is a good spell for removing enchantments. Fire at the Expert level is useless; none of the skills affect multiple targets, and Blindness and Berserk can be cast quite well anyway, as the duration depends on the strength of the magic. There are many other decent spells, but they are used only if the above-mentioned ones are not available, out of desperation. In my opinion, casting various force fields, etc., when you already have Blindness, Resurrection, Slow, Haste, and Berserk seems pointless.
I develop my hero to be approximately 70% warrior and 30% mage. My choice is EARTH and AIR; if I happen to learn Water, I won't be upset, but Fire can go to the trash.