Ziggurat

Pretty basic, kill 500 enemies that are poisoned, flaming, frozen etc.

I found this easiest with a poison staff. (You can check your equipment by pressing when you don't have a level up waiting for you).
Pop an enemy a few times with your staff until it's poisoned. (It'll have a green tinge to it). Then finish it with your wand.
An easy way to set this up is to press to select the staff, then press to select your wand. Then use to quickly alternate between the two.

Hope this helps. Game on! =D