In this guide, I will explain my choices behind this build, and why it is the most effective one out there.

Brief rundown on why I chose these runes:

Diana is an AP auto attacker. She isn't total spell oriented (as there are other champs who scale better with AP for abilities), but she also isn't purely auto attack (as there are other champs who have better cc for auto attacks). Instead, she is a hybrid, and quite the lethal one too.

Quintessence: 3 AP
For any magic based champion, starting ability power is the best quintessence to get. Scaling AP is not cost effective, which is why I didn't chose it. Plus, the bonus AP she gets at the beginning helps her "W" and will end up shielding her more from monsters.

Glyph: 9 Scaling AP
I decided to go all scaling AP for glyphs because I felt that grabbing MR was unnecessary. She is an assassin, and building her with MR might help her a little, but isn't worth giving up precious AP late game.

Seals: 3 Scaling Armor, 3 Scaling Health Regen, 3 Scaling Health
Trying to decide what seals work best was very difficult for me. I have played her with just armor for seals, but grabbing all armor isn't very cost efficient. If you get scaling armor, it's very poor early game, and is decent late game. Scaling Health and Scaling Health Regen were the most cost efficient late game, and are pretty relevant to my Diana play-style. I want her to be able to sustain in jungle, and this combo of runes definitely does it. Mana runes would have worked too, but neither scaling mana, mana regen, scaling mana regen, or mana runes were cost effective.

Marks: 9 Precision
The combination of Lethality (with autos) and Magic Penetration (with spells) is perfect for Diana. It works so well with her precision mastery, as well as her auto-attack/spell play-style. Precision marks are a bit unusual to see in most champs, but I believe it's the best way to go with an expert Diana build.

Masteries:


Ferocity Tree:
Sorcery > Fury) It's better to go for the early ability damage over speed. Late game, you will not need the attack speed because your items will give you enough.
Fresh Blood) Dealing an additional 10+(1*lvl) for first basic attack is such a crucial point for Diana, and beats the other choices.
4 Natural Talent, 1 Vampirism) I would say to go all 5 in NT, but having that little bit of lifesteal has helped me in early jungle camps.
Battle Trance) BT is the only option I think fits my Diana style. I play her where I can jump in, fight for about 10 autos, and then kill them. Battle Trance just makes me do it faster.


Cunning Tree:
Savagery > Wanderer) The extra movement speed you get from Wanderer doesn't do much for Diana, as even if you put all 5 in the tree, you still couldn't fight scuttle efficiently. If you worry about movement speed, just build Boots of Swiftness instead of MP.
Assassin) I went Assassin over RA on Diana because I like to counter jungle. Plus, it helps me fight 1v1's better, which I tend to get myself into.
Merciless > Meditation) You could put 1 point in Meditation if you wanted, but I feel that with Merciless (3% dmg), Battle Trance (3% dmg), and Assassin (2% dmg), I can be very strong to take on.
Greenfather's Gift) GG is perfect for any jungler. It helps them take camps quicker, and really is good when you are counter-jungling.
Precision > Intelligence) Because I build Diana to an expert level, she has some hybrid MP and Lethality runes. So Precision just made sense to take.
Thunderlords) Thunderlords works really well, since Diana's passive already needs 3 hits to proc her ultra hit. With Thunderlords, you are just amplifying the passive. Plus, it gives you 10 + (1*lvl) dmg, just like Fresh Blood, except Thunderlords is even better and scales with AP and AD.