I think what Frost means is that if Poison brings your opponnent below 0 health you still need to finish your spell in order to win the battle which is kind of odd but still not a big deal IMHO.
@prawnie16: Spellcasters do get the Spell "Learning" which increases the experience you get from killing monsters by quite a bit. It really helps getting those levels faster because as far as I can tell the amount of exp you need is tripled after each level. Also spells never miss and with Regenaration which lets you heal HP every few seconds + Ice Arrow which slows down the monster's attacks you are pretty much invincible.
Also make sure your Wizard gets the spell "Learning" at the moment i'm at ~lvl 4400 with that spell and get 20-50 million exp depending on the monster i'm fighting. Quests give me like 60-100k.
"You're fighting a Ninja Unicorn" nice one =)
Oh and btw learning as a skill is WAY off. I'm getting like 1-2 million exp for a kill but only 60k for a quest. Got learning at 3k.