This guide is formed through the many games I have played with Diana and a few guides I have followed. I would consider myself an intermediate LoL player so I may not have too much credibility but this guide is formed based on my experience playing as Diana. Viewer's Discretion is Advised.

She is a Melee Mage Jungler that can also be used, mostly, and I can say very successfully as an assassin due to her skill-set. Being able to chase down, burst and keep her opponent close she is a very formidable champion.

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Pro's/Con's

Pro's

+Low CD's +High Burst damage +Effective in chasing down her prey using Crescent Strike + Lunar Rush Combo +Very descent survivability +Late-game multi-CS potential

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Con's -Mana hungry early on (use Crystalline Flask to solve this issue) -All-in champ with no escape options

There are only a hand full of spells that are actually usable out of the 12 that are provided. Sometimes I use Exhaust but the two hands down spells I use when rolling Diana are Flash and Ignite

Flash: As she does not have any escape mechanism this is pretty much your only way out of a nasty situation. Use it strategically, as it may actually be better to Flash towards your prey if your Crescent Strike is on CD.

Ignite: This spell has brought down a lot of champions to their knees in front of me. It is an excellent way to further whittle down those HPs.

I have also read that if one wants to jungle they better pack Smite or they would suck. I say you don't need it. Perhaps the nice 1-hit is good but later on when you can pretty much destroy any minion you want, even solo dragon, I don't see the point. If you replace either Flash or Ignite you'd be worse off in lane or late-game duke-ing it out. Smite does make jungling easier, no doubt about it but I would still prioritize Champion killing.

This sequence pushes for damage output as fast as possible, with only 1 point in Moonfall at the beginning, prioritizing it as the last skill to max. She is quite straight forward in this regard.

When jungling start with Pale Cascade, getting at least 2 points in it. So the sequence becomes:

Well I have experimented a bit and I have found what I believe is the optimal Mid Diana Item build:

Nashor's Tooth: Works well with Diana's Passive Moonsilver Blade, adding AS and AP with some CDR. What else might you ever want? In addition, with the recent item changes this item just became a Must-have from a possible item choice, at least for me, as they removed the Mana regen from the Fiendish Codex dropping the whole item's cost down!

Iceborn Gauntlet: To finish off that Sheen. I have previously advised to take Lich Bane but with this bad boy s*** just got real! Armor, AP, Mana, and Slow.

Rabadon's Deathcap: A staple for all AP chars. A must-buy. Besides, who doesn't want 30% more AP? You mad bro? They did drop the AP% down to 25, however they are raising it back to 30 at the cost of, well, increased cost ><. I personally would have preferred for them to return this item to its former glory of 140AP 30% but I doubt they will do that.

Sorcerer's Shoes or Mercury's Treads: Depending on the enemy team comp either go MPen or MRes. Goes well with either Red or Blue runes. Also take the Enchantment: Distortion add-on for the boots for that much needed Flash CD reduction, since Riot doesn't like this skill too much.

Farming is actually pretty easy with Diana. Together with your AA which results in a horizontal multi-hit attack every 3rd strike and Crescent Strike you will be felling minions like there is no tomorrow. I usually place myself in front of the melee minions and aim my Q such that it hits the mages in the back, and of course always try to harass the opposing champ.

After you have managed to max Crescent Strike and leveled to about 11 or 12 you can practically 1-hit multiple minions resulting in a very fast gold farm. Utilize her passive, comboing it with items like Nashor's Tooth for greatest benefit.

These are my runes of choice. You get your share of MPen, MRes, Mana regen and AP boost. The armor and AS runes you would swap out if you decide you want to go Jungle.

I want to keep it short and to the point so there you have it. Diana is one insane champ. Not to mention that she is powerful, albeit with some bad matchups but who doesn't have those. She is also hawt! Sure she can't be matched to Ahri but still. She has it all pretty much: the horsepower (even post-nerfs galore in the recent patches) with her Crescent Strike + Lunar Rush Combo, mobility to gank like a bawss with Lunar Rush and Moonfall, and the sustain through her Pale Cascade. Whatever does one want more? Whoever underestimates her will regret it.