Leona is an easy to use support that can easily get an ADC fed and win the game, 2.75 seconds in stuns is amazing and she can be coupled with just a select few ADC's, you can always use whoever you wish but the best with her is Corki he can use his built in flash move in, and be frontlines in bot with you. Other good ADC's are Vayne and Graves, pretty much any ADC with a built in flash is good with Leona.

Her passive activates with any spell that hits an enemy: Leona Marks her enemy with a sunlight debuff, any spell or attack from an ally champion will ignite this debuff doing 20/35/50/65/80/95/110/125/140 gaining additional damage every two levels.
Shield of Daybreak: Stuns for 1.25 seconds and does additional damage.
Eclipse: Leona gains 30/40/50/60/70 additional armor and magic resist and after three seconds it explodes dealing damage to enemies, if it hits an enemy it presists for another 3 seconds.
Zenith Blade: Leona throws a solar image of her sword, going through enemies and pulling her to the last enemy hit.
Solar Flare: Hits enemies in a circle dealing damage and slowing by 80% for 1.5 seconds, if enemies hit are in the center they are stunned for the duration.

Early Game:
Use your E to go in then Q and W and exhaust if the ADC is above 1/3 health. After the enemy ADC is dealth with the support will be an easy double unless its a tanky support, then they may escape.

Mid-Late Game:
If your ADC is fed then take a turret and roam, I wouldnt advise taking a turret before 10 mins because you can still get some more kills on the bot lane that will be key in carrying. After turret is down go mid and try to kill the mid or at least take a turret, if you cant PLEASE dont feed any lanes if they are fed as it is. If your adc isnt fed play safe and if your behind more than 1 kill dont go in, its a loss. Play safe, farm, poke if you can.

Pros:
- 2.75 Seconds in stuns
- Easy and fun to play
- Aggressive support
- Can get the ADC fed easily
- Stuns useful at all stages in the game
- Great engage


Cons:
- Can engage on wrong person
- Stun can be wasted if you do Con #1
- Ult sometimes glitches when it should stun it slows
- E range is not exactly the length it says it is

Im going to add more to this, please dont rage me complaining about how theres not enough, comment and advise me on various subjects and i will take your advice in account during my next update.