Make sure you hit level 2 at the same time or before Leona as her level 2 all-in is very strong. In the bottom lane, this is the first minion wave followed by 3 melee minions.
Leona’s roaming potential is much stronger than most Supports. If she is unable to kill the enemy in lane, she will look to roam. As soon as she leaves lane, make sure you ping your team and communicate that she is missing.
With good wave management, you can make it very hard for Leona to engage. By keeping the wave closer to your side of the map, you will make it impossible for her to engage with her E without diving your Turret.
Avoid fighting in areas like the jungle or around the Dragon/ Baron pit as it will allow Leona to get a really good Ultimate R off. Try and spread out and fight in a lane if possible.
When fighting, try not to group too closely as it will give Leona a really good Ultimate R and stun multiple people. Split up so she can only CC 1 or 2 people at best.
Leona is prone to poke and disengage. As soon as she tries to engage, try to disengage and create distance between you and her. Additionally, try to harass and poke her down with long-ranged abilities to make it suicidal for her to engage.
Leona’s level 2 power spike is very strong. Make sure you hit level 2 at the same time so you can trade back with her incase she engages.
Once Leona has Mobility Boots, she can roam with ease. As soon as she leaves lane, ping your teammates so they know she’s missing.
At level 6, Leona’s kill pressure increases as she can use her Ultimate R from afar to lock you down. Play respectfully and do not push up if you are without Flash.