The two key points to take away from gnomasaur's guide are:
1) Avoid getting close to any of the edges on the map as well as diablo's cages. Getting close to them will trigger diablo's teleport ability.
2) On phases 1 & 3, always keep yourself positioned so that one of the two large crystals is between you and diablo. This will keep you from getting hit by his flame attacks.
If gnomasaur's build/strat works for you, great. I just didn't seem to have the skill required to focus on dpsing diablo with magic missile while still avoiding all of his attacks. I also seemed to be lacking one more defensive spell. So for the noobs among us, do it the easy way.
Your only attack spell will be venom hydra. Hydras alone provide enough dps to kill diablo and your shadow clones while allowing you to focus all of your attention on avoiding diablo's attacks. All other skills apart from magic weapon are for utility/survival.
~30k hp, ~300 res. This in combination with blur will allow you to survive 2 melee hits.
30k+ dps (buffed). Dps isn't that important for this strategy. Successfully avoiding all of his attacks is far more important.
Having 25% movement speed is extremely helpful (required?? less might also work. i haven't tested that) because you can avoid diablo's melee charge attacks by simply running away. This saves you from having to use a defensive spell every time he charges.
Always position yourself on the other side of one of the two large crystals from diablo (in other words, avoid any direct line of sight between you and diablo). Use the minimap to help you position yourself when you can't see him. Always keep venom hydra up on diablo's side of the crystal. Make sure not to get too close to any of the edges or cages, as mentioned above, or he will teleport to you. If done correctly, you should be able to avoid almost all of diablo's flame attacks as they will simply hit the crystal and never reach you. In case one of them does end up flying your way, use slow time to avoid it. In case you do end up getting too close to one of the edges, avoid diablo's teleport->attack with diamond skin or teleport. When he charges at you, 25% movement speed should allow you to simply run away from him without taking any damage from his attack (use diamond skin or teleport to get away in emergencies). Obviously, avoid getting caught in any of diablo's cages. Try doing this while still staying out of diablo's line of sight. If you are caught in a cage, spam teleport. I was often able to get away from him without dying. Although very infrequent (doesn't even happen every fight), diablo will sometimes put a damage over time spell on you after he melees you. Try diamond skin/pot/healing well to survive. Once you get him down to 65% or so, the next phase begins.
Wait about 1-2 secs, then hit diamond skin, slow time, and spam teleport. This will get you away from your shadow clone who will spawn right beside you. Two of the clones can immediately oneshot you (there are 3 different types, but for the purposes of this guide, the differences between them don't really matter). This is why you have to put as much distance as possible between you and your clone. The clones seem to get kind of stuck when close to the fissure (as long as you're on the other side). So just kite, keep them at a distance, and position them on the other side of the fissure while venom hydras dps them down. Once your clone is dead, diablo's shadow clone will spawn. He is just like diablo in phase 1 except he doesn't do the 360 degree aoe fire attack. If he spawns right on top of you, hit diamond skin and get away. Use gnomasaur's strategy of always trying to keep diablo on the other side of the fissure. As in phase 1, use slow time to avoid fireballs, just run away from his simple melee charges (teleport/diamond skin in emergencies), use teleport or diamond skin if he teleports to you, and always keep hydras up. At about 66% and 33%, diablo will disappear and another wizard clone will spawn. Diablo disappears from the minimap right before the clone spawns. This will be your cue to use the same strategy as mentioned above to save yourself and kill the clone. After a clone dies, diablo will reappear. Dps diablo's shadow clone all the way down to 0%.
Same as phase 1. The only real difference is that diablo now also does a breathing flame attack. There is a distinct animation and sound before he does this. This attack can be avoided by simply running away from it (run sideways away from it; watch gnomasaur's vid to see how it's done properly). There are also some meteors that drop from the sky (they look like falling holy light) It's very easy to avoid them. Other than those two points, follow the same strategy as in phase 1.
Hope this helps. I'll try to get a vid up soon. Gnomasaur's vid should help a lot in the meantime.