Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
I just began playing ranked games in the middle of season 5, so I am relatively new to playing League of Legends. Ever since I acquired Diana in a random gift giveaway from Riot, she has been my favorite champion. Her combos are crazy, and once you get them down, nobody can stop her burst. With the new preseason patch, I had to play around with build path and mastery combinations, but I believe I have found the best path for midlane Diana. Take a look, and let me know what you think!
You could go with a wide variety of options for Diana's rune page. I go with AP quints because she is a burst mage, and these seem to give the largest return in game over other quints. Attack speed reds help proc her passive which aids in last hitting, split pushing, and making her auto attacks hit hard when your abilities are on cooldown. The mana regen yellows help Diana stay in lane a little longer early game and lets her dish out more poke with her Q. Standard magic resist blues help sustain against the normal AP champs that you will face in mid lane.
This is pretty standard stuff. I will give a few notes of importance. I choose Oppressor in the ferocity tree over bounty hunter because Diana's E gives a 2 second slow, and the Hextech Gunblade also applies a slow. While dishing out so many slows during her combos, the extra damage during these is a lot more beneficial than the percentage damage increase given by Bounty Hunter. I put 12 points in the Resolve tree because the cunning tree does not offer Diana much if you are not filling it out down to the Thunder Lord's Decree. The lower level masteries in cunning do not give Diana much. Runic armor in the resolve tree greatly increases the shield given from Diana's W. Insight combined with the cooldown bonus for summoner spells given by Lucidity boots makes teleport much more readily available for quick engages and sneaky plays.
Diana really cannot do much before hitting level 6. Play passive and focus on farming before level 6. Poke the enemy champ with Q while simultaneously using this ability to effectively last hit. Do not just hit the enemy champ with Q. Diana has low mana levels early game, and that will quickly deplete your mana pool with little payoff. Rush Lich Bane and hit level 6. Once those two things happen, the only thing left to do is effectively hit her combos and burst the enemy down. Her damage output once you hit 6 is absurd, and the enemy will likely not expect just how much burst you're capable of right at level 6. Mid game is all about continuing to farm and finish out the build path. When laning phase is over, continue to roam in the jungle and empty lanes to acquire as much farm as possible. When your team is ready to team fight, be there. You are the carry and will most likely be capable of putting out the most damage. Nobody can stop you one on one, but be careful of split pushing and staying in lane too long. The burst potential against one champ is there, but once your abilities are used, you are a sitting duck until they are off cooldown. The Hextech Gunblade and attack speed should help your autos deal decent damage until your Q and R combo is available again. Look for picks, continue to focus objectives (she can easily dismantle towers with her passive), and be there for your team when they start fights. Diana's power will carry you to victory. One important note: never engage a full health enemy with R without marking them with Q first. This will send your R on cooldown, and your burst will be gone. Only use R without landing Q first when you are finishing off a low health target. Diving in and placing your R on immediate cooldown will make you a target for easy pot shots and certain death.
Let me know what you think of the build path, runes, masteries, and anything else in this guide! Thanks for reading and happy hunting on the rift!