With this build i always get:

132 Attack Dmg
507 Ability Power
1.645 Attack Speed
432 Movement Speed
81 Armour Resist
52 Magic Resist

At lvl 18.

Im not sure why but I get different stats than what mobafire is telling you. Try the build out, its my favorite for diana.
The trick with this build is too try to get first blood by being in one of the side lanes. She could also be used mid, but I prefer sides. Early kills will make your diana a power killer from the very beginning.

These are the stats i get with my build. This time I bought deathfire grasp instead of Athene's unholy grail. They are both good, so just take your pick!

The runes are very helpful. This mastery and item set up does not focus on the growth of diana's mana. That is where the runes come in handy. Diana's build is enhanced through the choice of buying the greater seal of clarity.
By buying 9 of these runes we are able to spike diana's mana regeneration. At lvl 18 it is very rare to be completely out of mana.

The rest of the runes focus on her AP. She has potential to monstrously powerful, these runes help achieve that goal.

Her masteries are helpful in enhancing and bringing out the max potential out of diana. This mastery tree focuses on her offense and defense. In order to take on 5 champions, this diana needs to be able to withstand combos from multiple champions as she quickly picks them off.
It is mainly focused on attack because the best way to survive with satisfying life, is by killing the opponents quickly. To support her damage intake defense, I put 9 mastery points in her defense.

The item choice is very important. This build maximizes diana's killing efficiency and enhances her multi kill power.

The berserkers greaves are cheap and can be bought quickly. This enhances one of diana's main offensive attacks. Her basic attack. Choosing these boots will give you the edge when your in mid fight and all your skills are in cooldown.


Rylais Crystal Scepter is very important in early game because it instantly makes diana a tough killer. If you have this amount of life in early game diana can virtually 1v1 anyone, it also intimidates foes so it lowers the likes of being ganked by single, lone champions.


Contrary to what many might think, ionic spark is one of the main devices to create a power house diana. This item has allowed me to get so many kills. If you in a multi battle, and you have a champion in low life, they try to run away. Since this item enhances attack speed and shoots out a lightning bolt to 4 champions every 4th attack, champions can't outrun the lightning. The lightning has killed many running foes, even when I don't focus on them. Attack speed also takes advantage of diana's passive. She has a very powerful passive attack that occurs every 4th attack. At lvl 18, It is more powerful than her skills. This item choice is a great choice.


When i first started this build, I began with Deathfire's Grasp because of the cooldown effects and nice ability power. However, I realized Athenes Unholy Grail provides more AP along with the same cooldown effects. Only down side, Athene's Unholy Grail is more expensive.



Either of these items are a great choice.


I believe rabadon's deathcap is essential to any AP character. I personally play ahri as my main, and rabadon's death cap +140 AP is an amazing stat boost.



Trinity force provides the final touches and pulls everything together. It has everything diana builds and enhances every aspect that diana needs to be enhanced. Diana's Ability power, Attack power, Attack Speed, health and mana. Everything this build tries to focus on, trinity force supplies. With this final touch, Diana becomes a lovely penta killer.

This skill sequences focuses on her main attack,
crescent strike

pale cascade.

The skill sequence gives diana a way to protect herself while she is an offensive player. If you have low life and are running away, pale cascade might save you. The focus on crescent strike will also help kill champions quickly.

The summoner spells are made to ensure the death of champions.
Flash will help reach champions, so you could use crescent strike, then combo it with her ultimate, and her other moves.

Ignite will help in finishing off champions in low lvls (and high) when you lower their life and they are trying to retreat back to their turret, where they think they are "safe."

Diana has amazing combos for farming.

If there is a large group of minions tanking to a turret, these combo is amazing for to them all at once:
To gather them


To kill most of them

To pick of the stubborn ones.

If you don't to waist mana, just go right in with your basic attack. Diana's passive, and ionic spark will quickly kill all the minions.

Pros

+She can give destructive damage, even though she is based on ability power
+She can be a very effective early game killer
+Long range attacks combo nicely with her attack damage
+Her cooldowns are very short!
+Amazing at turret diving

Cons

-She looses mana quickly if her skills are in constant use (mostly in low lvl)
-If a player wants to keep a distance from enemy, he only has one move
-She is squishy when she is low lvl

This build creates a diana that is able to withstand all 5 champions for a short period a time, enough time to kill them one by one. I recommend you only take 5 of them on when you feel confident in this build, and diana! If you don't really care about getting multi kills, this guide is also meant for massive amount of kills. Good luck.