Leona is a very aggressive support specializing in engagement and peel. Vision control and movement speed are key to using her early-mid game to catch people out of position, while tank stats are needed to be the main tank in the late game team fights.

Leonas early game is really dependant on the players ability to know when to initiate trades with the enemy bot lane. Being on the same page as your adc is important so that you don't dive in and not have your adc back you up.

the mid game or reliant on Leonas ability to find good picks and good engagements to get the team a good lead. Establishing good vision control is important for both engagements and picks so a sweeper is a good idea after you have your sight stone.

Late game is where Leona needs to engage, hopefully landing an ult on multiple members, and then peeling for your adc once the fight has begun.