This is my first guide, hope you hate it dear haters. I first started playing Leona yesterday building her like some nimwits are trying to build her here. It was slow, i got nearly no kills, was bored as hell... but then i got smart!

I noticed all her spells were AP additive, so i started experimenting.

So if you don't like this build ok, just remember this one thing! DON'T FORGET TO ADD A LOT OF ABILITY POWER

ps: when i'm talking about AP its ability power
ps2: have fun :)

pro:
hot as hell
pusher
insane damage
insane survability
nice combos
area of effect
Crowd control
farmer (just use eclipse)

cons:
I don't think this works on 3vs3 (didnt try yet)
You will think of twilight the whole time (sucks A LOT)
She will get nerved obviously
Beginning is the hardest, if you suck here you're doomed (but stay close to the towers the first lvls and you will be okay)
Maybe not for starters and i advice you to do some co-op vs AI to know her better

I just take magic pen (obvious you want to make her burn and melt everything)
Some armor and magic defence (she's still a tank)
Quintenceces: more magic pen

Summoner Spells: I just use ignite for the extra damage in the beginning (get you more kills and faster stacking). And teleport just to be back on the battlefield faster or help others out because you are still a tank somewhere :)

I kind of did the usual, only not for a tank. :D
I added more offence, to boost it just a little more... and then yeah it's more fun you get kills and a nice build faster.

I choose to start with the classic AP book. This gives you that little extra to start with (to boost your Eclipse Spell). When you have some gold left overs, just buy potions, you never know they sometimes save your life!

The second thing you should buy is that pendant. It will make you recover faster, and you need it anyway (for the warmog).

After that i'll usually finish the book, and try to build my stack. (it's ok if you only get assist first, you just need to get a stack and try not to lose it).

Depending on the situation I buy boots or that red crystal. Buy the time you have your Warmog armor your stack should be 10 or more. Now you are starting to make serious damage.

To add some more damage I add the other book, gives you some extra CDR and by the time you have a stack of 20 your total CDR will be maxed (40%). That way you can harras them big time :).

This is the basic build, the rest depends on you. You can add Banshee's Veil for that little more protection, or start boosting your speed with nashor's tooth or trinity cap.

What I usually do is adding some extra AP, making you even more powerfull, you are becoming a real big palladin/warmog. For ex: Rabadon's Cap. And Malady's Pink Dagger (gives you extra stackable Magic Pen + extra Magic Damage mmmmm :) )

When you are facing auto-attack people buy the icy-shield instead of the second book I was talking about.

I think she has an amazing combo! She could be Jarvan's girl. ( I think the Jarvan with Leona force could be a deadly lane combination. Just imagine Jarvan Ulti + Leona's Ulti...)

Facing one opponent:
Just hit A and Z, that will load your Eclipse spell and making things ready.

2 options:
He's running away
-> Use your E spell to get close to him, by the time you're there you shield will explode, giving you a lot of damage and when you are hitting him with your auto attack you will make extra magic damage due to your A spell and you will stun him even more. If he's not finished yet just ultimate him to death. If he's still alive, just to everything once more. Doing this combo properly will give you more something like 1500+ damage every 10sec.

He's charging you
-> He's a dead men. Keep hitting A and Z spell eventually a R(ultimate) if he has less then 400hp. When he gets how deadly you are he will start running away, and you know what to do know ;).

Escape moves:
You have 2 CC moves: your A and R spell. What i do is usually put on my Shield (Z spell), gives you extra armor and magic resist. (fun part it's offensive defence)
When only one champ is chasing you, use your A spell. Start running and if he get's close, hit him, and run away. I had some games where I start running, but after using the A en Z moves I start running after them. And they never outrun you with your E spell + CC moves. (if you can aim correctly)

First levels: I stay close to the towers or if I'm lucky and I have a good lanepartner is try to stand by his side, CC the ennemy, and get plenty of assists to start with. And once I starting to have my stack he's becoming my assistent.