Decent passive skill for a tank. It allows your teammates to deal higher dmg.
In order to use it effectively, you will always have to jump into a fight and activate this skill with Eclipse. Your teammates will take care of the rest.
Melee stun, powerful if used correctly
Use this skill to deter enemies chasing ur teammates, cancel out enemy champ's ulti (such as MF's ulti)
It is crucial that you'd stun the squishy champs like annie or ashe, not those
'i-never-die-u-sucker' champions like Alistar, garen or morde unless they are chasing ur almost dead teammate
Your bread and butter. Not only it enhances your defence greatly, but also it deals AOE damage arounds you activating Sunlight on targets near you.
You should also try to stay by the enemy when you use this skill as the duration of your defence buff will increase if it hits enemies.
This allows you to start the fight. You will dash in the enemy group, press W, stun the squishy champ and boom.
Also you can chase enemy champ with it as it roots an enemy champ.
"Artillery Attack" Huge range and capable of stunning/slowing the enemies caught in the flare. This skill needs tons of guide that I will describe it further in "Skilling Order" section.
Skilling Order
Master Eclipse first as it makes you tankier and easier to activate Sunlight to nearby enemy champs. Zenith blade is only needed for it's ability to root the target and dash. So you only need it at lv 1.
The effective tactic for Leona is to hide in the bush, activate Eclipse, wait few seconds and stun the target when it is about to burst. Alternatively, you can also activate [spell=Clipse], wait few seconds and use Zenith Blade to the target and boom.
This video shows an example of this tactic.
Now as aforementioned, Solar Flare has multiple usages in various situations.
It has very short cooldown, so do not hesitate to use on a single target.
1)For most of times, you will be find a running enemy champ with only an inch of health left. You can easily last hit with this skill as it has huge range. The important thing to remember is that you will anticipate the enemy's path and lay ur ulti ahead of them.
This is an example
As you see, I was able to stun the fleeing fiddlestick with my ulti. Make sure you lay ur ulti ahead of enemy champ as I did in that video.
2) If one of enemy champ is preparing to or launching his/her ulti on your teammates.(Such as MF's ulti or fiddle's ulti or nunu's ulti) You can easily cancel out their ulti by stunning them. (Make sure you aim your ulti right so it would stun the target, it only stuns the target if placed at the center of Solar Flare). This one needs a bit of pratice
3)You can help your fleeing champ with your ulti. Place Solar Flare ahead of chasing enemy champs. If you put it right, it could stun them or at least slow them.
Sometimes you may have to wait for enemy champs to waste their flash so your ulti won't miss.
As a streotypical tank in LOL, Leona is slow. You will need speed to lead your party.
Also Leona gets mana hungry easily without any mana regen items(which I don't get them often)
Last one may looks interesting or awkward to some people, but Greater Mark of Insight greatly enhance the efficiency of Leona's ultimate skill. Solar Flare
It lets you last hit the running enemy champ because of it's broad range(Do not disregard the potent of this skill) There will be tons of opportunities where you can last hit squishy enemies with ur ulti
Summoner Abilities
Mandatory= Flash
You can start the fight by Flash jump in the target, Shield of Daybreak on the most deadly dmg dealer. Also for obvious reason, you can run away with it.
Recommended= Exhaust= this is great spell as it basically cripples an enemy champ. With your Zenith Blade , Shield of Daybreak and exhaust you are a one deadly champ Ignite= This ability alongside your passive Sunlight is great combo for enemies. Teleport= This is the one I always get as I mostly move around entire map to aid all of my teammates and gank, but if you want to stay in a lane feel free to ignore this spell.
Others are not recommended. "Not recommended" is a gentle way of saying "Do not F***ing choose these" by the way.
Items
Some of the typical Tanking Items
Anti-AD
Randuin's Omen= Very effective against AD champs. But I would buy this item if enemies have more than 3 ad champs which not many would do. Make sure you put this item in your first item slot so you can easily activate the AOE slow whenever possible.
Frozen Heart= Another good anti-ad armor. It is relatively fine item for some tanks that desperately suffer from mana loss. But Leona won't be mana hungry if you get mana regen runes like I didNOT RECOMMENDED FOR LEONA
Thornmail= If you get focused by annoying AD champs or their GP got fed so much. This item is on your list. It's cheap and grants you tons of armor.
Anti-AP
[item=Force of Nature]=grants nice amounts of magic resists but gives you 0 health. Leona needs health. NOT RECOMMENDED FOR LEONA
Banshee's Veil= Probably the best ani-ap armor out there for tanks. Some magic resist with health and mana. Plus the ability to block enemy champ's skill makes you fearless to rush in.
Quicksilver Sash= Very cheap, decent amounts of magic resist, and ability to cancel CC.
This item depends on the situtation. If you find yourself being CCed too much, you may buy this item in the beginning to counter. But I generally do not buy this item.
Build Example
Now there are multiple item builds you can go.
First of all,
If you are a super novice, or first-time playing Leona
Able to disable enemies' ultimate with Solar Flare
Very tanky even without items
Able to last hit running enemies
Leona herself is a CC machine
Cons
Very Very hard to farm
no escaping mechanism
Quite hard to use for beginners
quite dependent on teammates although most tanks do(for exmaples it's really hard to play Leona if your teammates just cowardly crawl behind as you rush into a fight. This is something you have to overcome with. There are just some weirdos who won't support their team.
Working in the team
I want to say this for all tank users
1) You are the Leader of your group
2) Thus, you will always be at the front, leading your teammates
3) You will always initiate the fight, if you are afraid of starting the fight you lose
4) If necessary, you will die for you teammates since DPS are more important than tanks.
5) You will always aim for enemy DPS or carry not their tank
6) If you choose to buy DPS items instead of Health/armor/magic resist items, go uninstall League of Legends. You just don't have an ordinary brain to play AOS games.
I'd like to say this because I occasionaly find some people very inappropriately playing their role as a tank.
This is how you fight as a tank in your team. Don't be afraid to fight because Leona is a lady.
She is a deadly tank. Always at the front in a team fight.
The introduction of using ulti in mentioned in the above section so I will assumme you guys understand by now.
Conclusion
When Leona first appeared in this ferocious LOL world, I first had an impression like "lol, can she even stay alive?" and 2hours later, BAM ! She has been successfully placed as my main tank in the ranked match instead of Alistar. I can dare say she is the best tank in LOL if used correctly. Also her Solar Flare kind of gives you a feeling of sniping a target in FPS. (you wait for your enemies, anticipate their path and BOOM! headshot!)