I was really struggling with my mage at the beginning, but after a restart and different approach I find them very fun to play.
The key to making them usable at least in early game, is to spread out their points. Get a point in hydro, aero, pyro, summoning, maybe some damage reduction as the new AI knows who the weak one in your party is. Also, gear will come with random +1 to different skill trees, i.e. geomancer, etc... So use that to augment your characters so you can do different types of damage.
The funnest thing I have found so far with my rogue is: Rupture tendons on their melee character, cloak and dagger away, then watch the melee character bleed out as it moves across the screen.
When building a character, pick whatever class you want to role-play, then add some magic or physical damage dealing skills to them via skills. I put geomancer on my ranger and will give him a point in pyro as well. Also the necromancy skills do physical damage (I think most of them do, can't remember atm) so I give that to my mage.