Pros:
*Strong late game.
*Can kill most champions 1v1.
*Fun to play.
*Looks cool.


Cons:
*Squishy early game.
*At the first you will have hard time aiming that Crescent Strikes.
*Later in the game you will go kamikaze trying to kill all the enemy team, just because you are fed.

I take Flash, because you can simply Flash+ Lunar Rush and you secured a kill. I can't stress out how good this spell is, but you probably already know.



Early game use ignite to finish your enemies. Mid-late game use it to shut down heavy healers like Dr.mundo, Vladimir and even Volibear. This spell is useful trough all the game.

Other viable spells:



Take this if you are new to diana, or just you want to be in a lane longer.



Ghost is a great replacement for Flash, but well Flash is Flash and nothing can replace it for me.

This item is going to give you the early game mana regen+health+AP, which is really needed, because of the fact that you are a melee champion in mid. Stack few of those if needed.




Extra magic will come in handy against that Shen or Malphite. It will also melt down carries who don't stack MRes.




This item is going to give you some needed HP, mana and AP. Plus the passive is good too, so grab it early so you can stack it.




Some more health and AP plus the slowing passive is just great. You benefit from everything.




Item which is a must for every single AP carry in this game. Nice AP + a 30% increase in your total AP is just great.




This item will give you some extra attack speed, which is good on Diana and some AP. It is very useful.




Extra magic penetration against tanks is very good. By now noone should be able to stay alive against you.

At levels 1-6(early game)
Try to farm with basic attacks and use skills to harras the enemy champion. Don't go aggresive until you get lvl 6. Try to get Doran's ring, boots and start you Catalyst the protector



At levels 6-16(mid game)
Now that you got your ultimate you can go aggresive. Do your main combo - Q+W+R+R+E+ Ignite if needed. By the end of this phase end you should have build your Rof of ages and started your Rabadon's Deathcap.



At levels 16-18(end game)
Now you are super tanky damage dealer. You are probably full-builded if they didnt surrender. Go in, do your combo and get the kills. Diana is one very strong champion end game and you should take this to your advantage.

Moonsilver Blade(passive):
Diana gains 20% additional attack speed. Every third strike cleaves nearby enemies for additional magic damage.


An AoE steroid for your basic attacks. When the mid game comes you will be doing around 300 aditional damage with just basic attacks. And thats alot and you will melt squishies away.



Crescent Strike:
Diana unleashes a bolt of lunar energy, dealing magic damage in an arc. Crescent Strike afflicts enemies struck with Moonlight, revealing them for 3 seconds.


Your main abilitie. Use this and then use your ultimate for 0 sec on your ultimate. This deals a lot of damage so you can use it for harass, or for clearing big minion waves faster.



Pale Cascade:
Diana creates three orbiting spheres that last up to 4 seconds and detonate on contact, dealing magic damage to all nearby enemies. She also gains a temporary shield that absorbs damage. This shield is refreshed if all three spheres detonate.


Think of it as Leona shield combined with Ahri's Fox fire. When maxed you will have around 300 shield and if the balls detonated it will double. Nice, ah?



Moonfall:
Diana draws in all nearby enemies and slows them for 2 seconds.


Kinda like Darius' Apprehand but its a AoE and it slows. Max it last.



Lunar Rush:
Diana becomes the living embodiment of the vengeful moon, teleporting to an enemy and dealing magic damage.
Lunar Rush has no cooldown when used to teleport to an enemy afflicted with Moonlight, and will consume all active Moonlight debuffs.


One of the best ultimates ingame. When you use it, your enemy will have 1/4 less hp. And you can use it twice. Just an overall awesome ultimate.