Leona playes verry passiev and try's to use her relic well until the first cannon minnion coms when the cannon minnion dies from ur relic basic attack u will hit lvl 3, from lvl 3 leona can start playing rlly agro and punish there ac or support for miss placment and give away first blood to your adc.

early game leona is a decent tower diver when maxed W and stacked chain vest early.
Mid game leona Destroye anny one's with her cc burst and the damage coming from ehre team
Late game : leona jus tcant die annymore ur the biggest tank around with your w and utlity