Why runes like this?
Becouse diana is an Assassin and in my opinion is the best mix between sustain and dmg.
She is a champion that like to jump on the enemy and melt him down in a seconds.
So runes like this will give us a nice sustain and damage in early and in late game as well.

Like with runes, we need to deal damage as fast as we can and survive this. So masteries like i presented are in my opinion the best way to do this, also good choice will be give 3 points in Vigor and 1 in summoners Wrath on offensive tree, making 22/8/0. Also magic pen in masteries and rines are important becouse it will decrease lack of magic pen in item build.

Why Flash and Ignite and what is also good?
So flash will help us in defensive and agressive situations as well, will keep us out from danger or it will help us to engage.
Ignite - when enemy team have a champions who likes spell vamp or life steal it will help us much, becouse while ignite is on enemy we should have time to melt him down.
Also Exhoust (for enemies that like to deal a lot of damage) or Teleport (for improoving our assassination on squishy teams) will be a good choice, but when we pick up teleport we need a good map awareness.

Why items like this?
Becouse most of them give us Ability power or damage and sustain, which is important for squishy assassin champion.
- DFG will criticaly improve our burst damage.
- Zhonya's will give us a nice boost of armor and ability power as well, plus its passive will help us to survive in critical moments.
- Abyssal Scepter will decrease lack of our magic pen, and will give us a magic sustain, that should help us to burst down AP carry of enemy team and will help in battles with tanks.
- Lich Bane is a great item on melee ap champions, and with Diana it works great becouse of her passive, we will be abble to deal a lot of damage just auto attacking enemy, plus it gives us a small boost on sustain
- Guardian Angel (will be great against team that have a lot of damage and CC, also will do a great work with good used Zhonya's) or Hextech Gunblade (when enemy team is squishy but have big damage comming out from them, it will give us a spell vamp and life steal, plus will slow chased enemies)

We should not be over agressive under 6 level, we should farm as much as we can and try to harras enemy with our Q. When we reach level 6 and we have DFG we should be abble to burst down the enemy, we must remember also diana dont have to much running ability to warding your area will do a great job.
Our skill sequence at attacking should be like this: Land a Q on enemy -> then turn on your W -> Jump to enemy with R and if he will be trying to run just launch E, after this if enemy is still alive launch on him another Q and R, menwhile using you DFG, now the enemy should be dead :)

After reaching 6 level try to roam as much as you can, but have in mind to keep your lane pushed and safe, becouse killing enemy and loosing the tower wont be a good exchange. Also remember that your E can do an giant work for your team, just be sure you have enough sustain to jump on the enemies. Roaming should be targeted on major squishy enemies like AD carry or AP carry. Diana with DFG have so much burst damage to melt down most of the enemies, but dont forget that she is still a quite squishy champion, so dont go for suicide missions. REMEMBER also Diana like to take a lot of mana in early game, so be sure to have at least mana for your Q+R combo.

PROS:
- HUGE burst damage
- Can melt down almost all enemies in 1 v 1 situations
- Enough sustain to farm well, and safely roam
- Mobility
CONS:
- Dont have escaping abilities (only shield)
- Still a quite squishy
- Can be quickly killed in 1 v 2+ situations

Diana is an asassin champion with huge burst damage, with combination of sustain and damage she is great to melt down lonely enemies in seconds, her mobility give her opurtunity to chase enemies faster than her.
MUST REMEMBER the best way to use you R when enemy have a mark from Q on him, becouse then our R colldown is immediately refreshed.
Diana on this build can play on Middle Lane or Solo Top lane, and she will be able to melt down most of the enemies in 1 v 1 situations, even tanks.


From me:
Sorry for my english, and if I writed something that someone dont understand.
This is My first Guide so feel free to write what is good and what is wrong.
CHEERS SUMMONERS! And have a good time!

Larmind