If you are playing as a good character, dwarven infantry pairs really well with mages, they have huge magic resist and kick a lot of ass. Dont be afraid to toss in a few points of cleric for the healing and buffs, and if you are running an evil party, necromancy is decent to take as well, a mage heavy army works well with some expendable skeletons and zombies as cannon fodder. The Chaos spell makes a shambles of any large enemy force, it'll be awhile before you can cast it though. Just like in DnD, wizards start off kinda weak, but they escalate in power dramatically, soon enough you'll be taking out garrisons by yourself practically, and no fort can stand against a line of high level mages hanging outside the walls and nuking everything. Dont wear regular armor on a mage it weakens your casting, although iirc boots and gloves are ok. there is also twilight armor but it is very expensive. Once you have some money, either buy better mage clothes, or you can buy mithiril mail from 4 ways inn, or elven mail from elf forest, both of which give a small casting bonus while still being good armor. all without fighting a single battle, as you go from town to town trading and recruiting companions etc).

grab a couple mage recruits too, and you'll find you level up quickly just killing bandits etc with the magic missile spell (alternatively, you can make a surprisingly decent amount of startup money as well as renown and even right to rule by using the bard skills. If you are playing as a pure mage, you should get some robes, hood and staff asap.

How to use magic in phantasy calradia