Leona's all-in style of gameplay heavily revolves around jumping on your target and killing them as fast as possible. Aftershock's explosion damage further pushes your ability to kill the enemy after CCing them.
Engage your target, kill, and punish them by dealing massive amounts of damage to the tower while they respawn.
Conditioning on Leona is great once you've hit the 10min mark as both the bonus Armor and MR contribute to your [W] strength.
Unflinching's slow and cc resist empower your ability to stick to your target after engaging onto them for follow-up CC and damage.
Biscuit's on Leona allows you to take aggressive trades early while still being able to sustain the incoming damage.
If you pop a biscuit mid-fight when you are below 50% health, the extra sustain will allow you to absorb a ton more damage.
Cosmic Insight's well-rounded utility empowers your ability to chain CC the enemy and peel for your carry.