Use this build: Wizard - Game Guide - Diablo III (Could swap Paralysis for Glas Cannon with enough crit/speed)
My stats while killing him: 2.61 attacks per second, 34% crit, 120% crit damage and 26k dps
Strategy: Venom Weapon seems to be bugged and triggers Critical Mass a lot, which allows you to spam Frost Nova all the time. You will deal A LOT more dps when you're close, so fight your shadow from low distance in phase 2.
You should try to get attack speed/crit on every item slot and alternatively some chill on hit, but it's not necessary. Andariels Visage is a relatively cheap head item for attack speed and a fast main hand (~1.7 aps) + good offhand (7% crit/200 damage) should give you the dps.
It might take a few tries and especially phase 2 of diablo can be a little tricky. When it starts, rebuff immediately and use Slow Time to keep the projectiles of your shadow away from you (most of the time you die instantly before you can dodge). Try to keep him in your Frost Nova then and simply kill him, but make sure your Frost Nova is up when he dies, as Diablo will soon come back and you'll need it then.
This method can easily be used to kill other rares/champions. Pony level could be a good place to farm.
Video of a Diablo kill by some chinese guy