Make your Wizard lawful good, load up on protection spells. Make no aggressive actions until he starts attacking you. Let him fall for attacking a non-hostile, non-criminal, good bystander...Then kill him.
Then he rolls in with a paladin of slaughter who also happened to take the full mage slayer line.
Out of doubt, out of dark to the day's rising
I came singing in the sun, sword unsheathing.
To hope's end I rode and to heart's breaking:
Now for wrath, now for ruin and a red nightfall!