Diana is an incredibly strong AP champion. In order to get the best out of Diana you are going to want to play smart early game. I usually go with my team into the bottom bush off the start of the game. I don't invade because by invading, you give the enemies the first few attacks, leaving you at an incredible disadvantage.

As far as Runes go, I'd say to get the Greater Mark of Scaling Ability Power, Greater Seal of Scaling Ability Power, Greater Glyph of Scaling Ability Power, Greater Quintessence of Scaling Ability Power. The Regular (not scaling) Ability Power runes, are better for early game, but are not needed as Diana is awesome. If you feel that Diana is not awesome, kindly go f*ck yourself.

Recommended:These runes will give you 83.16 Ability Power at level 18

For masteries, go for 21, 9, 0. You are going to want to get all the AP you can.

Start the game by unlocking your Crescent Strike or Pale Cascade, I usually start with the Crescent Strike but either one is helpful. If you unlock your Crescent Strike first make sure you have it availible when a weak enemy champ is running away. One easy shot and +1 kill for you! If you have your Pale Cascade unlocked first then use it to initiate the early-game team fight. If will do a nice chuck of damage to the whole enemy team while giving you a shield, as well as a second one if your three spheres connect with an enemy.

When laneing against an enemy that you wish to attack, initiate with your Crescent Strike, quickly hit your Pale Cascade, and then your Lunar Rush. If the enemy starts running, press your Moonfall, thus bringing him closer to you and slowing him by 35%/40%/45%/50%/55%. If the enemy is still escaping, restart from the beginning. ( Crescent Strike Pale Cascade Lunar Rush)

Tips:

• Make sure that before you use your ultimate ( Lunar Rush), that you hit them with a Crescent Strike if at all possible. Doing do will remove the cooldown on Lunar Rush.
• Remember to use your Pale Cascade when fighting, as it will give you 2 shields and damage your enemies severely. Also, it has splash damage. (Like Crescent Strike)

Pros:

• Extremely high damage output
• Has escape methods ( Pale Cascade, Moonfall, and Lunar Rush)
• Able to spam ULIMATE
• If enemy is hit with Crescent Strike then Lunar Rush, Lunar Rush has NO cooldown
• If enemy is not hit with Crescent Strike before being hit by Lunar Rush there is only a 25/20/15 second cooldown on Lunar Rush

Cons:

• Semi-squishy early game

When trying to run from battle, there are 3 different methods you can use with Diana to escape.

1. Your first option should be your ultimate, Lunar Rush. Use Lunar Rush to jump to an enemy minion, or jungle camp.
2. If that doesn't work or is on cooldown, use your 'W' Pale Cascade. It will give you a small shield, and a second one if the 3 spheres hit enemies.
3. If they are still on your tail, hit your 'E', Moonfall. Be careful using this as it actually pulls them into you. In order to use it to escape, while runninig away press it, and keep running. This will slow the enemies by 35%/40%/45%/50%/55%.

If all else fails THEN you use your flash, if you didn't equip flash then you probably should have.