Introduction
First of all i want to introduce you into my guide.
It's my first guide so pls be kind with me ;)

So... Why choose Hybrid Diana instead of Full AP?
Well of course the burst damage of a full AP Diana is higher but in my opinion Hybrid Diana is better in Teamfights that last for a longer time. For example if you engage on the AD-Carry you often have to burst your ult twice to instant kill him. As AP-Diana your dmg isn't that good for the rest of the fight because your Ultimate is on cooldown. If you choose hybrid you can deal way more dmg with your passive thanks to your attack speed. So you are able to deal nice dmg even without using your spells.

+ Very high burst damage
+ High constant damage with passive
+ Nice lifesteal and spellvamp
+ You can check Dragon and Baron with your Q (grants vision)
+ Hybrid so enemy tanks have to go armor and magicresistance
+ Makes fun to play

- not as squishy as a AD-Carry but still squishy
- gets focused alot
- needs a bit training to hit your q
- very expensive build

Well i wont explain much about the masteries but i'll say a few words to it.
As you already know Hybrid-Diana needs Ability-Power and Attack speed.
So most of the points i spent is just this.

The other points i put in Resistance for a better defense against AP because you go mid and most mid chars are AP, so it helps you to stay alive longer.

I spent another point into Summoner's Insight because it reduces the cooldown of your Flash. This 15 seconds can secure a kill or save your life in dangerous situations.

Runes

Greater Mark of Attack Speed
Well, i choose this marks because they give you a huge attack speed bonus, and this is what you need to activate your Moonsilver Blade as often as possible.

Greater Seal of Ability Power
This Runes should explain themselves. Most of your damage comes with your AP so... the more AP, the better!

Greater Seal of Magic Penetration
Magic Penetration is very important against opponents with huge magic resistances. Even if your enemys dont have any items for magic resistance there is still the basic resistance that blocks some of your damage. So if you go with this runes your damage will increase even if your enemys have no magic resistance items... and that's what you need -> Damage Damage and Damage :3

Greater Quintessence of Magic Penetration
Look at the seal explanation ;)

Well your Summoner Spells depend alot on your way you play.
But in my opinion Flash is a must have on nearly every champion.
You can argue about the second Summoner spell but i think the true damage of Ignite is a good choice to secure kills in early and lategame.

I think the Skill sequence of this guide is very similar to a all other Diana's (excluding jungle).

First of all you skill your Crescent Strike because it is the skill that deals most damage. You can poke with it and you can afflict your enemys with moonlight to jump on them with your ultimate.

Your second skill you are maxing is the Pale Cascade. It shields incoming damage for a few seconds and hurts alot too. You should activate it before you start to jump on your opponent.

Your third skill you max is your Moonfall sure it is good but you are a damage dealer and because Moonfall deals no damage your other skills are more important.

Well i think i dont have to say that should skill your Lunar Rush whenever it is possible, it's a great skill in combination with your Crescent Strike. You can jump out of combat if you push your enemy to his tower and there are creeps behind you, so it can save your life too.

Now about my item choices.

First of all you should start with Boots and 3x Health Potion.
It's the most common starting setup because it is very effective. Boots gives you a huge boost into your mobility and that helps you to position yourself better in combat. Because you dont have any other Bonuses on your starting setup like dorans ring would have, you deal a bit less dmg and are a bit squishyer. But hey thats why you got your Health Potions!
They allow you to stay on lane for a longer time so you can farm succesfully and get fed.

When you go back the first time you should have farmed enough to get your Sheen. It's a cheap item and it increases your damage alot! When you Lunar Rush your enemy you should automatically add a auto attack to add your sheen proc for the increased damage. If you wanna burst a bit more you can hit some creeps twice before starting your Combo so your Moonsilver Blade procs at the beginning already.

When you got your Sheen you should already have a good damage output for a while. That's why you can build your Rylai's Crystal Scepter now. Start with buying the Giant's Belt because it grants you alot of health! Rylai's Crystal Scepteris a good choice because you often have to engage as Diana. That means you jump into 5 Enemys and hope that your Team helps you as fast as possible. Because of this you need a bit HP. ( Rylai's Crystal Scepter is very effective in combination with your Moonfall.)

After you got your Sheen & Rylai's Crystal Scepter you should get more ability-power to increase the damage of all your skills! And what item is better than Rabadon's Deathcap to increase your ability-power alot? Right, theres no better choice! So get it after your Rylai's Crystal Scepter.

Your Damage with this build should be already very amazing and you should be able to kill the Squishy enemys in a few seconds. Now the hybrid part of this build starts. Get your Nashor's Tooth it increases your AP, your attackspeed and decreases your cooldown! In other words it's a perfect item for Hybrid-Diana :)

Your Hextech Gunblade is the next item. Sure it is very expensive but it worth it!
It got a huge amouth of livesteal, spellvamp and ability power! And don't forget the active ability of it! It can secure a kill for you or can save your *** :D

The last item is the Trinity Force. So why choose Trinity Force instead of Lich Bane? Well Trinity Force grants you a bonus to nearly all of your stats. In combination with your Rylai's Crystal Scepter all of your attacks slows your opponents with a chance of 50%. Isn't that amazing?

Well when you reached the point of the game when you got all this items ( in most items you wont get so far) you can buy Oracle's Elixir to destroy the wards of your enemys so you can control the map better! You should get the other Elixirs in this order. 1st Elixir of Agility 2nd Elixir of Brilliance and as last item Elixir of Fortitude.

In earlygame it isn't always easy to farm as Diana. Sure your passive helps you to get the creeps fast but your autoattacks got no range so you can get poked easyly. At the earlygames it helps to do two hits on the melee minons and hit all caster minions with your Crescent Strike and your Moonsilver Blade.

Sometimes there are huge waves of creeps. The fastest way to get em all is to walk in the mid of them, activate your Moonfall to get all on the same position and cast your Crescent Strike and Pale Cascade.

Well in lategame Diana is able to kill nearly every opponent. But her main task is to hit the Crescent Strike on the heavyest damage dealer (most time AD-Carry) and burst him down as fast as possible. Most time this should work really well and you can help your team to get the other enemys too, because their main damage is missing.

Well i just want to thank you to read this guide and i hope you enjoy playing Hybrid-Diana.
Feel free to comment and rate my guide, but please remember it's my first one :).

Best regards
Redsun