1) Buy yourself a Lilith's Rod (VERY CHEAP ON SOME SERVERS)
2) Melee campaign mobs to earn combat xp
3) Enfeeble the enemies while melee-ing to earn enfeebling xp
4) Cast your most powerful nuke at the enemies that are near death to earn kill-shot xp
5) Heal campaign allies to earn healing xp
6) Skill chain to earn skill chain xp
The reason why you wanna melee with a Lilith's rod is that it has a additional effect that drains mp. It's a 33% chance that the effect will activate. Since most of time your not actualy casting nukes your mp will be pretty high which you can use to cast cheap enfeebles instead. Please note that enfeeble's that resist count but the one's that have "no effect" don't. Also MOST of the mobs have mp except for the one's that come out of Belfry's.
The greatest problem that mages in campaign go through is their mp pool's. Resting to recover mp is a waste of time and xp that you could of gotten by just meleeing with lilith's rod.
Lilith's Rod Auction House Page