The thing about the Horned equipment is that it penalizes your Agility, which is actually an extremely critical stat in this game. But if it's the best you've got for those characters' respective equipment slots, I suppose it'll have to do.
Do you have any spare Shrine Keys? With them, you can get Leonardo's, Leah's, and Maximus' best weapons: God's Hand, the Spear of Destiny, and the Seraph Halberd. I don't know what section/s of the Elemental Shrine you've cleared, but God's Hand is found on the Agility path, the Spear of Destiny on the Intelligence path, and the Seraph Halberd on the Strength path. In fact, might you already have any of them in your inventory? Please check.
I see you replaced Ezra with Jessica. Was he not doing well as a healer? Jessica is low-level by comparison. At that level, I don't know that she's viable for the final battle. What's more, she's got the Lotus Charm equipped, which lowers max HP/EF... and at Level 50, she's already severely deficient in those stats compared to the others. If you're going to use a character besides Ezra, consider Maximus. See what equipment you have for him (maybe tell me if you want). His Charge ability really helps him dish out the damage.
I'll ask again: Do you have Stat Raisers, Element Barriers, Life Guards, and Inhibitors? Spider Webs, Dark Tonic? The latter two are limited in quantity, but the first four can be bought in the Castle in the Sky. Leonardo's Spirit Fire and Omniwall can do the same things as a Stat Raiser and Element Barrier, respectively, if you don't have them or run out of them.
As I see it now, your best attacker at this point is Romeo, having the best weapon (Excalibur). Do you have any accessories that could further pump up his Attack and/or Strength? For example, do you have the Ogre Badge? You get it by returning all of Ben's statues to him. Even the Battle Scarf gives him a bigger boost to Attack than the Air Rune (though the Rune covers all of his weaknesses, and Cugei unfortunately plays on a lot of those). Check to see if you have the Emerald Crown for Romeo, as that will give him another layer of protection against his weaknesses.
Leonardo is really meant to be a support character in this battle, with his three new skills. Unless you have God's Hand, he's probably not going to be dealing a lot of damage.
Did you return the three relics of the Zodiac to the Castle in the Sky? If so, which piece of armor did you choose? They all provide Elemental resistance - extremely valuable in this battle. If you haven't already done this, I suggest taking the Metatron Armor and giving it to either Leonardo, Romeo, or Maximus.
Do you have the Hobo Lily and/or the Union Ring? What about the Black Leather Corset? All three of these combined will turn Leah into a death machine. Any one of them alone may help a bit on the offense (though you might sacrifice some Defense in exchange). Not to mention the Spear of Destiny, which is very easy to find (just keep taking the right path in the Intelligence part of the Elemental Shrine, and unlock the door with a Shrine Key if you have it).
I realize it would be very inconvenient for you to have to return to the Elemental Shrine and go through all those battles again... I hope maybe you have some of the other equipment I mentioned here so you can tweak your party and make it through before having to resort to that. But if you do have to do that, I would suggest using the opportunity to level up another character, such as Maximus, and maybe even get the Seraph Halberd for him (I'll direct you to it if you decide to do this). As well, get any of the equipment I mentioned that you don't already have (I'll tell you how to get everything if you need me to).
But hopefully, you won't have to resort to that. Good luck!
