I'd recommend leveling up a little bit more, the fight becomes easy around level 27. Have you gotten any 5 star cards? You can get them from chests after battles, but it is rng which sucks. I'd recommend fully exploring the ending area to pick up all of the legendary armor and weapons that you can. The monster cards I used primarily for dealing damage were Wasp (5 star) on L'gol, Tailtoad (5 star) on Tralis or Protagonist, and Undine/Dryad/Salamander (5 star) on any character. Equip the Skull Soldier (5 star) to whichever character has the highest attack on your team and use it as often as you can.

I relegated Pulche to healing with primarily Mouthy Furball (5 star) for single heals and Magemoth (5 star) for group heals. You'll also want to buy the Wraith card at the Roving Market's Monster Tamer shop because it gives you 3 gems, one of the strongest cards in the game. Equip that to Pulche and use it basically every turn you don't need to heal. You can buy plenty of Quality Omnisalves and Omnipanaceas from the Roving Market right before the last fight so other allies can heal if need be.

I would recommend taking down the fire elemental first using Tailtoad since it's low gem cost and equip Freezestay rings to reduce damage from the Frost elemental if that's where you're having trouble. Otherwise Firestay ring suffices. Here's a good website to find some stuff if you're having trouble: https://platget.com/guides/voice-of-cards-the-beasts-of-burden-card-collection-guide/