Mages are great damage dealers and thanks to mana shield they can even take quite some damage! IMHO the best playstyle for a mage is just to keep a bit of range, and try to react to what the others are doing. You deal damage, they tank and do their combo things. Whenever someone kicks them into the air, you can jump up and air-combo them. When they land just send over the Ball Lightning and most of the times that will even be enough.

You can also go a bit more head on, with bosses i.e.
Since you (probably) deal the most damage on the boss, he will almost certainly come after you, thanks to mana shield you can (if you have at least a little bit of armor) kinda 'tank' on the boss! the only problem is that the boss will probably get so close, that you will start hitting with your spear... there goes your damage

For PVP i've noticed Blizzard is an excellent skill. I always try to do it like this(Dragonica Gold):
1. Blizzard (freezes them)
2. Ball Lightning (more dmg!!)
3. Down+Attack!

this will surely hurt them ALOT!