I feel like Leona should be good against immobile mage supports like Zyra, Malzahar, MF, etc.
But she's not. Why? Because those champs do so much damage in both poke and all ins that Leona literally never has an opening against these champs without jungle help. Even if you manage to keep your health up until you reach lvl 2 or 3, you still lose when you go in because these champs do so much fucking damage.
TL:DR Buff Relic Shield/Nerf Spellthiefs.