A powerful exploit is to select an item from your inventory so that it stays with your cursor, and then click the options button, and then click anywhere on the screen. The item disappears, but it's bonuses stick with you permanently. This essentially allows you to equip infinite items.
I found Mage to be extremely effective, and practically unbeatable by melee units.
Use dazzle to stun an opponent, blinkstep to teleport away, and keep hitting him.
With multiple enemies, you can do the same thing - dazzle one, blinkstep away from the other, then use Arcane pulse to teleport all enemies to the dazzled one, effectively putting you on the opposite side of the screen, ready to dish out massive damage. Repeat. You never even get touched.
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