Hello and welcome to my guide on Diana! By using my experience using Diana I will be attempting to guide you through some of my personal beliefs in what a strong Diana should build and how they should play, or maybe aide you in creating your own build which suits you.

For my Tanky DPS Diana I tend to go 21/9/0 with standard AP masteries in Offense while taking magic resist and extra health in Defense to help you survive against other mid champions however if I decide to go extremely damage heavy I go 21/0/9 Simply swapping out Defense for Utility in mana and mana regen so you can spam Q and harass until you can either get a gank from your jungler or until you hit level 6.

For runes I take the same runes no matter which masteries I am using as I find all of the bonus stats my runes give me invaluable especially health per level.

Tanky DPS
I start with the standard boots and 3 health pots although you could chose magical tome and 1 health pot. I then straight go for boots, now I don't know how many of you saw my item list and though "WTF!? Berserkers greaves!?" Yes, Berserkers greaves. You can choose socerers shoes or berserkers greaves, on the tanky DPS I would chose berserkers because as you are built reasonably tanky you can ednure long fights, so you need something to fight with when your skills are on CD, berserkers greaves procs your passive nicely and your passive has a 0.6 AP ratio which is insane combined with its base damage. Rylai's is my first built item for the slow, AP and Health, an all around good item on any AP champ in my opinion.
Rabbodon's deathcap again a great Item on any AP champ that is a large damage dealer, Diana benifits greatly from it as even her passive's damage is increased aswell as her Q and ulti. Finally Rod of Ages and Abyssal scepter are great tank items which grant AP and I chose Morello's thanks to the CDR extra mana regen and extra AP.

Assassin
The items are simillar however the boots become heavily dependent on the enemy team and its composition, berserkers greaves aren't as important because with this build your more reliant on your initial burst of damage rather than doing portions of damage over time. Although berserkers greaves is probably better if you follow this build because it will proc Sheen, Guinsoo's rage blade and when you build Sheen into Lich Bane.
Rylai's is there for the same reasons as the tanky DPS. Rabbadon's is more nessecary than the tanky DPS for the obvious reason that your damage output is key. Next build Sheen for the damage proc and in preperation of Lich Bane (should the game last long enough). Guinsoo's is a item which I found to work well on Diana not so much for the extra AD that's just a nice side effect, but rather for the AP it gives, along with a stack of extra attack speed and AP for each time you cast or basic attack, aswell as this can lead enemys to not suspect your damage output as it will keep rising as you battle them, this is an item which I will definatly consider buying at a earlier stage of the game.

The skill sequence prioritises your Q as with most builds, aswell as increasing the damage your shield can take. However in the Assassin build I take my E rather than my W sometimes for the sake of sealing my enemys fate should they try and run.

Standard flash, ignite although ignite can be substituted for teleport if you need to get back to lane quicker.

Pros
High potential burst damage
Reasonably good ratio's
Shield which damages enemys and can refresh itself
Ulti with good damage and either no or low CD
Very good CC tool
Passive which does TONS OF DAMAGE!

Cons
Low movement speed compared to some other champions which can make escape difficult
Somewhat squishy without tanky items
She is melee so you can struggle early game if in mid lane
Can be prime focus if the enemy's have any sense.

Farming with Diana as previously stated she is melee so tends to struggle without ganks by the jungler, if you are getting heavily zoned, as with any lane is call for a gank from your jungler as farm is crucial for Diana or else she is quickly nullified from being under farmed and/or fed.

Well, this guide has come to a close, I hope I was able to either guide you to a better experience with Diana or help you in creating your own unique build to trash your enemys! Thank you for reading.

- Flare Demon.