That is a super hard match-up. We need to see the outcome of Yuno's attack. Meroleona's new mode got more panels.
Speed: Mereoleona > Yuno and this is fucking impressive since Yuno has the element that should make him one of the fastest characters in the series later on.
Mana pool: Mereoleona =< Yuno.
Raw power: Mereoleona > Yuno.
AP: Mereoleona =< Yuno.
Mana Zone: Mereoleona = Yuno.
Magic Element: Yuno = Mereoleona but here is a tough question. Is Lava stronger than Spirit Dive Wind?
I know that if it would be straight-up Fire vs Wind they would be even but since she's straight-up using Lava/Magma whatever and Yuno is using Spirit Dive Wind I think Mereoleona =< Yuno.
Overall: I think Mereoleona takes this but it's not a low diff by any means. Mid high diff I think.
I give her the edge mostly because she's Captain level character. Yuno definitely has Captain level capabilities but he needs more feats.
@Admiral Lee Hung What do you think? Especially about the part with Element comparison.