Why should you take my advice?
I was bronze in s2, i played 620 games of leona in ranked and am now plat 1 omw to diamond in s4.
Leona is my favorite champ because she has potential to make plays and she is super fun when you can carry your AD to victory or even solo smash their ADC! Sometimes your ad can suck... in which case you roam mid and support the AP carry or jungler! It doesnt matter who is fed when you play leona as long as somone on your team gets fed and you get those assists and win those objectives!

Pros
-Easy skill shot landing with MS
-Fast Lev. 2
-Lane sustain/ hard to gank
-Hard Lockdown lev. 6
-Easy Pusher with W
-Excellent mid game team fight potential
-Hard to Kill
-Progressively gets tanky as the game goes on
Cons
-Skill reliant
-knowledge of the game reliant + how to handle matchups
-Adc reliant
-Team followup reliant
-Must be a very AWARE PLAYER

In team fights you assess what is best to protect team or all in engage the enemy?, I base my decisions like this:
1. Is the APC or ADC on the other team fed? Goal: Lock them down and kill them no matter what
2. Is MY APC or ADC fed? Goal: Get engaged upon and use everything to protect. via/baiting ext, or engage when vision and opportunities allow them self for free kills.

As Leona you want to keep control of the map and your LANE! Only spread to mid lane if you have killed their tower, they are NOT pushing hard enough to take yours, And you have secured Dragon or put vision on it.

My style of play with Leona is WIN lane HARD or go home. You can and will carry the game if you just stay in your zone and apply massive pressure.

Always try to Q before you E, this is why i love MS sooooo much and why i go 0/9/21 as leona.
Getting in and out of fights is a skill all to its own, your W should provide you enough armor and mag resist early along with your runes to provide you lane sustain + you now have targonz for 50+ more HP and 4 health pots dont forget. You can be the BULLY! Dont get poked to below 70% at lev. 1, but DO get poked to 70% not stupidly but when minions are about to die to harass the **** out of the enemy ADC. I know its really annoying, but thats what you gotta do to win games. Eventually the adc will come in to last hit and you walk forward behind your minions for an easy E Q Combo.

Probably the most important skill as leona is knowing when to go in with your E. Knowing that, you should always be constantly looking at where their Jungler is at.... a good rule is if you dont see him top he is probably in your lane about to gank yours.
If you are pushing.... ward and if you are past half way.... be careful about going in with your E or tower diving unless you know you will win the trade



Early laning phase is the most important as Leona, have your ad proc your sunlights passive, 9/10 if he procs E and Q with a couple auto's ignite will finish them off, even so you will create lane dominance and auto win your lane if you didnt get harassed below 70% like i said earlier.

Always pop your biscuts when you take more than 2 auto's, remember the passive on the biscut gives you instant HP so popping them all at once might win you the trade in a long battle!

Smart decisions when to go in is all that matters. Separate your skills for max damage output, ward in and around next objectives. Never go out into enemy jungle alone! Get Kills and B, don't overstay or get baited :P Game knowledge comes as you play more and more so don't get frustrated.

Raging does nothing, always look for what you could have done better given a situation. There is always room for improvement till you have mastered the ways of the Sun Goddess.

Objectives > Kills, you wanna win games, you go for all the objectives on the map- Pressure, pressure, pressure. Never stop.