This is my pure tank build for Leona. I have had great success with this build. Only issues is; Leona is a tank. If she does her job correctly. Some squishie AD or AP carry will survive and you'll hopefully still be alive to roll the other team. This is my first guide, and I have no clue how to do the fancy thing with the images, so bear with me.

Pros:
Can initiate.
Has survivability.
Is a freaking brick wall.
Very fun to combo.
About 10 second skill cooldowns.

Cons:
If you're a k/d ratio person, this is not the champ for you. She's a tank, she gets assists, but since she's in the enemy's face, she will get alot of action.
If you are bad at skill shots... you will not have a good time.
If you are bad at last hitting... you will have a very bad time.

Standard tank runes:
Armor yellows
Mdef blue
Apen reds (easier to last shot, hit like a truck early levels)
Movement Quints (They can try to run from you)

0-21-9
I usually grab flash instead of ghost since you can "soft" reset aggro on people and get right back into the fight.

Start with regrowth and 1 pot.
After first time back, get philo and boots. DO NOT COMPLETE INTO MERC TREADS.
The reason you get a philo is for the mana and health regen. Longer lane, better items faster.
Also buy: pots and wards.
After second time back, attempt to finish sunflare cape, and then finish boots. (should be around 15 minutes)
If you experience problems, you might want to get giant's belt and then finish boots. It all depends on how well your game is going.
I usually get Force of Nature next, followed by frozen heart.
At this point, you should have a ridiculous amount of armor and mdef and it WILL take their whole team to take you down. If your carries are doing their job, you will win.

I max out the shield first, BUT I start with 1 point in stun. This is crucial. Leona's stun is awesome and will stop enemies from owning your allies. At level 1 it can mean first blood or not. I then get Zenith blade for the initiator skill. Level 2 you can be hardcore harassing your lane with that and the q skill. I usually pop e to initially stun the enemy, and then pop q to stop the enemy from running away. I then try to max shield out as fast as possible, but I also put another point in e because you hit for more. I abuse Zenith blade, so it works. Then I max out Zenith blade, and finally the stun.

I grab flash for offensive and defensive options. You can save yourself from dieing after the initiating fight. You can also flash into a fight.

I grab exhaust because it can save lives; Not just yours, your allies.

I happen to find, that you can 3 man Barron at level 12 with her. But first, you must get sunflare cape and merc treads.

There you have it. A brick wall. Leona comes in, and stuns and if the enemy focuses you, they will have a very hard time killing you. Even if they get a black cleaver, you will have 300+ armor with your shield up. You should also have 100+ hp/5 generating, which is just so funny. You can leave a fight with a sliver of hp, and come back a few seconds to save the day with your stuns.
This is what you can expect.