First off you HAVE to have the arcane university unlocked.

MAGE:
1) create a spell under the spell class you would like to level up and make the spells duration 1 and its magnitude 3. 3) cast the spell repeatadly and you will gain levels like crazy.

Assassin, Theif, ECT...
1) Sneak: go to the arcane university and go to the Praxographical center, go up the stairs but do NOT enter the other room, instead rig your controller to sneak and walk into the corner and your sneak ability will raise in levels.
2) Marksman: After gaining access to the horse whom goes by the name Shadowmerre take your bow out and attack it. (note: that shadowmerre can NOT die) it will then become "knocked out" when it comes to simply do it it all over again.