Tactics for Dungeon.

If you listen to you, you can't level up mages at all...

You can, it's just riskier on a large map; a fighter is much more reliable. But in general, it all depends on the template and the map (on many fixed maps, it's preferable to level up a mage rather than a fighter, as the red orb and cloak are disabled, and there are many places to replenish mana).