This is my build for Diana as an AP Off-Tank. I am sure many many people will disagree with this build but I have had a lot of success the past few days seeing as Diana seems to get focused A LOT late game. This build I feel gives incredible survival with a good amount of damage output.
Passive Ability - Moon silver Blade - Gains attack speed(per Level) and every third attack does magic AoE damage. I feel this is an amazing ability for Laning and getting in those last hits early when coupled with Crescent Strike.
Q Ability - Crescent Strike - This is an awesome poke ability. Does some decent damage as it shoots out in an arch( from Dianas' right hand toward location ) dealing damage to all in its path. Not only does it do moderate damage it also leaves "Moonlight" debuff on targets it hits showing location for 3 seconds which is extreamly nice for checking brush. Keep in mind when using this ability to lane if u have a line of mobs running at you to move up or down to get them to form the arch so u can hit them all at once.
W Ability - Pale Crusade - This is your defensive ability. Spawns 3 orbs that deals a moderate amount of damage and provides you with a shield. IF all 3 orbs hit a target you then get another shield. Keep this skill in mind when you are about to use your ultimate to dash in at your target as it will damage and keep you somewhat safe during small engagements.
E Ability - Moon fall - Pulls all targets in and Slows them. The slow on this late game is crucial. This will keep those pesky people that use flash from getting to far away. The main downfall to this ability is that is has a small radius.
R Ability - Lunar Rush - Dashes toward your target dealing damage. If "Moonlight" is on your target then it does not use your cooldown. This ability is your Game Winner so to speak and is extreamly versatile in its uses. casting your Crescent strike on your target using lunar rush and your shield is the main wait I tend to kill people. As your Crescent Strike adds moonlight to your target allowing you to use lunar rush for free the first time. Then your shield from Pale Crusade keeps you safe for your brief engagement and then you can use lunar rush again for more damage or to get away if it is early game but rushing toward an enemy minion.
This build will not only throw your opponents off( most expect a melee squishy Diana right now) but it will also give you huge amounts of survival. Late game and Mid-Game you can use Diana to lead the charge in taking down enemy turrets and their base. I feel Tank Diana is the way to go because she gets the "Target her first" syndrome in most games.
This guide is unfinished. I will return at a later date to finish it up. Seeing as this is my first guide I want to think things through on how to present the rest of this guide.
Thanks for looking and Good Luck Killing.