No idea about WoW, but - maybe depending on subjective semantics - Buffy's flirted with the spiritual side of kicking peoples asses, so I'd say monk.
That said, if there are any scythe/axe-thing lookalikes, then warrior would be apt, too. She, and other Slayers, have kicked all kinds of ass with that thing [http://a3.ec-images.myspacecdn.com/images01/59/a4848d41e7b637f0116e40260accc133/l.jpg].