Leona is one of the most tanky supports champions due to her high base stats and high defensive stats from her W. The amount of CC she has is very high, but she isn't exactly the easiest support to play.

Pros

[*] Very tanky
[*] Huge amounts of CC
[*] Huge range on ultimate
[*] Spells easily combo'd
[*] Lots of damage
[*] Scales extremely well

Cons

[*] CC can be dodged
[*] Somewhat difficult
[*] Gets harassed easily
[*] No sustain
[*] Squishy when W is down

There are many different possible mastery setups for Leona. Some examples are 1-15-14, 9-9-12, 0-9-21, 1-17-12 and 1-4-25. I personally take 1-15-14 masteries most of the time. As an agressive support, you should focus on defense and utility masteries, but you can very well take some offensive masteries too.

Extra defense stats and AoE makes maxing W the clear best choice.The goal with the way I max Q and E is that they both have the same cooldown. This allows me to stick onto targets by having the lowest possible cooldowns on my two spells. If your team's goal is to focus tanks, maxing Q immediately after W is acceptable. This way, the stun comes up faster and the E is only used to go in after the tanks are dead.

Armor is the preferred choice for marks. Leona needs to be tanky early.Quintessences are great support runes. They give you a good amount of gold, which you need because you shouldn't get kills or farm. These quints will basically give you about one free ward every 4 minutes.

Exhaust:
Exhaust is just the preferred summoner spell and helps Leona to completely lock down a champion in an engagement.

Heal:
Against an AoE comp or long sustain fights, Heal is an excellent alternative.

Ignite:
Ignite is surprisingly very strong on Leona too. With Ignite, a full combo with an AD with guarantee kills in most situations.

Leona is a strong support champion with a huge amount of CC. Her damage scales extremely well with levels and has incredible potential in skilled hands.I think Leona should be played more often and I hope this guide inspired you to play her. She is really underrated in my opinion but very fun to play and really strong.Thanks everyone for reading my guide. I hope you liked it!