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Ideally all 4, of course, but that will only work on a large map. After all, when playing as a mage, you must not forget that Wisdom, Intelligence, and preferably Magic need to be leveled up at some point, along with these 4 schools... On a large map—no problem, and basically on L too, but on M or S you simply won't have time. And in general—in my opinion, on S and M you should pick a warrior, and throw all your efforts immediately into scouting, gathering resources wherever possible, and active replenishment of several armies (neutral structures, Diplomacy skill): every little bit helps, distribute them among the players, and give the strongest and most numerous to the main one.
Sorry for the slight OFFTOP, but here lies a small part of the answer as to which magic is better:
smaller map—Air and Water.
larger map—Air and Fire
And without EARTH—you'll get nowhere, no matter how you slice it )))