Leona is one of my favorite champions to play. She is a very strong champion in lane. Her tankyness help to protect the adc from the enemies. She also has a lot of cc, so it is easy for her to initiate a fight or retreat safely. In the early games, she can use her E to approach the enemy adc and lock him down with her stun. In the late games, she can either initiate or turn over a fight with her ult. Due to her tankyness, she can draw lots of damage from the enemy team.

Red and yellow armor runes are great for Leona because it increase her tankyness in the early game. Even if the enemy jungle decide to gank bottom, it is likely for her and adc to retreat under the tower safely. The scaling magic resist runes help her with the late game from the high burst of enemy mid laner. The health quints increase her overall tankyness for her to stay longer period during team fights.

This mastery is a standard mastery for support player. It games some tankyness to Leona and some utility like mana regen and gold earnings. You can choose different masteries based on what kind of role you want to play. However, I found this mastery is best for me.

The ancient coin is a very reliable fit for Leona. She gets the bonus gold along with extra health and mana regen. This help to harass the enemy adc in lane. The item also provides a considerable amount of gold in late games. After the first time getting back to base, you should consider to get a ruby stone for the health and prepare to upgrade it into a sight stone. The boots help you to get back to lane faster and roaming around other lanes. The finishing build provides lots of armor and magic resistance, shield, and movement speed for allies. It help the team to engage or escape.

Her W gives her significant amount of armor and magic resistance, especially in the early games. Therefore, it is important to max level her W and harass the enemy team. Max her Q second because it provide longer stun time and allow your teammates to have more burst onto the enemy. Her E is a great way to get close to the enemy. She only needs 1 level of E in the early games. Her basic combo is to use her w to reduce the damage, then use E to approach onto the enemy member, stun him with her q, and at last, use your ult to lock down the enemy again. Sometimes the enemy is to far to reach by Leona's E. It is good idea to use her ult first and then use the combo. Be careful to not go into too far to the enemy team where your teammates can't back you up.

Pro

Very tanky
Many CC

Cons

No damage
Very hard to land E
No sustain

Leona is a very strong champion in the new season and really fun to play. If you can master this champion, you can carry the game.