Do it on the zamorak monk under the stairs right near the entrance to varrock castle. He has high mage defense and seems to shake off the times you actually don't splash pretty easy. Even if you hit a spell you can cast another usually immediately or 1 second later.
Go to limb ridge and go to goblins, stand north of the fence and cast one by one on all of the goblins, they'll slowly stand infront of you and the fence, then swap spell and repeat and maximise XP, much better than the monk.