This is my build for Leona, a champion I think many underestimate as an excellent team fighter who is very good at peeling and targeting enemy champions. She is very effective in high poke and high aoe teams with her passive. Hopefully this guide will shed some light on her true prowess.
Pros / Cons
-Extremely good armor buff makes her item independent early game
-DoT passive can turn her lane very aggressive depending on lane partner
-Peels enemies well
-Boatload of cc
-Weak health stats (a bit)
-Poor damage in spells and base attack
-Not a babysitter
I generally rush Ghost as mandatory, with exhaust being a preference over ignite. For Leona, I feel ghost is much more effective because it allows for better positioning to catch enemies than flash. Flash is the stronger escape spell, but that's not why I'm even considering it. Leona is a tank, so she should be the last to leave a fight if possible. Other possible spells you can get are Teleport and Clairvoyance. All preference, but straying from the ones I listed is not recommended.
For runes I run what I generally do on tanks. Only thing I would sub out are the seals for health/lvl, quints for movement speed, and blue for flat cdr. So it is all preference.
I go 0/21/9 just to make her early game tankier than 0/9/21. Either is fine though.
You generally want to max her Eclipse first, due to its AoE damage and armor/mr buff that wrecks enemies in lane. You only need a point in Shield of Daybreak since its main purpose is to stun and the duration doesn't increase as it gets leveled up. Zenith Blade is maxed 3rd just because it is capable of dishing out more damage. Her ult, of course, is maxed whenever possible.
My item build looks a little bit funky, but one has to bear with me while I explain why.
Usually Leona works well in a duo lane so her passive can be taken advantage of, so I usually run early regrowth to rush into a philosopher's stone. Merc treads may have to be built very early on if your lane is having some trouble. Heart of gold I find is totally optional/preference, I would only build it if I had some gold left and it is still early game (~9min). Anyways, your core before mid-game teamfights should be a philostone, merc treads, and glacial shroud.
Now mid game is totally dependent on enemy team. If they are AP heavy, I build a Wit's End. AD heavy, I go for a Sunfire Cape. These two items are very effective if the champion can stick to the enemy well, which is what Leona is very good at. The Wit's End will actually let her do a good deal of damage with the on-hit procs while raising her magic resist. Sunfire cape speaks for itself, it lets her stay in fights longer and continually dishes out AoE damage. If you have trouble though, I would skip Wit's End for a bveil if your fights are looking sour throughout. You want to finish Frozen Heart and have a good deal of magic resist, health, and armor. Combined with her W buff Leona becomes very hard to kill.
This is around the >30 min mark. Now you want to be tanky enough to peel and stay in fights, so you definitely need a great deal of armor, health, and magic resist. The Guardian Angel is extremely effective if you somehow get killed first, because the enemy team has to spend 2 seperate worth of burst to kill the tank. Madred's Bloodrazor serves the same purpose as Wit's End, mainly good to deal damage while you are sticking on to an enemy. You should have sold philostone by now, but you can make it into a Shurelya's if you like it.
Leona is a very good peeler, as in she can stop enemy champions from attacking yours with an on-command stun. Her E also interrupts, very useful against channel-ult champs. In teamfights, you generally want to be the initiator/counter-intiator. You can activate your W and then E an enemy champion while your team follows, or lead up with your ult. Counter-initiating is really just baiting the enemy team, where you lead them into your team while your W is active and your spells are off cooldown. Something extremely important to note is that Leona is very team-dependent. Leona has very weak damage, so do not be stupid and initiate fights your team can't win. You can usually tell this by comparing your team's items with the enemies, or see what items are on your team's champions. For example, if your team barely finished their core while the enemy has core+ luxury, initiating a full-on 5v5 is idiotic. In these cases, you want to draw the game out while your team can farm up, OR get vision on the map and catch the enemies when they are lower in numbers.
I plan to make this guide a little more in-depth with specific explanations on items and such, but for now I listed out the core of this guide. Please upvote if you liked this guide, or at the very least read before you give it a thumbs down.