using lightning

Lightning is so quick, that you can kill an enemy wizard before he even knows you're in range!
Using the GUI only, you can select the lightning icon and, as soon as a wizard spotted icon appears on screen, a few quick right-clicks of the mouse will cause multiple strikes the wizard in the world.
This approach, when coming into a game after losing a life, means you can sometimes kill an enemy wizard even before the FOW clears...

Lighting is also far cheaper than meteor shower, but doesn't have the ranged damage. However, this is because it is pin point accurate, and you can take out enemy troops, god like, without injuring your own troops in the melee.

defeating lightning

Run away!

Seriously though, you can't hide from lightning - it can be cast indoors (!) and you won't be able to escape it from hiding under a tree :(
So you're options are to get the wizard out of the area using haste, or teleport, or just keep healing yourself with heal or by eating food.