Each class must equip various spells and abilities for offensive, defensive and tactical abilities to be effective.

Mage: Fireball does a lot of damage, but charges slowly. Use Arcane Shot to do damage more often. Frost Shield is an effective defense. Chain Lightning can affect a large portion of the level.
Rogue: Mutilate, Shadow Cloak and Killing Spree will work well.
Warrior: Use Strike as your standard attack. Green and Purple are usually the least used of all jewels. Dodge is a good way to defend. For your tactical ability, drop Power Strike when possible and upgrade to Crow Swarm.