Being the foundation class, the Mage is a powerful one no matter which sub class (or promotion class) you choose afterwards. The Mage is a ranged attacker (or damage dealer, depends how you like to call it) and does AoE damage, which means that it will attack multiple enemies at once.

It works very well on its own, as well as in a team. It is not super tanky however, so you need to constantly be on the move and try to avoid as many enemy hits as you can, because the Mage was not made to take hits (that's the Warriors' role).

The Mage has a skill which allows you to get out of a sticky situation, and that's especially because the class can die pretty easy if you get hit by multiple enemies or by a powerful one.

Skills:

Ice Storm: Summon a blizzard for a period of time which deals damage and slows the enemies continuously in the given area. It also has a chance to freeze the enemies.
Frostbird: Summons an icebird which flies across the battlefield, dealing great damage to the enemies in its path.
Blink: Dashes forward in a gust of howling wind, dealing damage to nearby enemies and freezing them, while also healing the caster.

Since the Mage has a combination of both of the promotion classes' skills, it gives you the opportunity to try and get better with it, because it is super good! It will be super powerful, no matter which promotion class you will choose.