Admittedly, ultimate challenge and stuff is pretty difficult, but it's more about strategy than power-
I think what I used was probably a counter evasion build on the MC. (Just use equipment to get 100% counter chance, make sure one of your accessories is the ant shell shield, and stack as many additional counters as possible) Plus the deck of cards that allows you to cast regeneration and zombie on enemies so that healing over time kills them by shaving off 5% of their HP per round they take.
Ant shell counter build is built for defending against extremely powerful enemies even if you're under levelled, as you can survive as many hits as you can counter until your next turn, only thing you gotta watch out for is making sure you have enough counters until your next turn to survive.
Also I think I remember I had a build on Ray to give her both unlimited fury form, and unlimited...celestial? I can't remember the exact items/forms as it's been ages, but Ray has an armor that gives her one permanent transformation, and an accessory that gives her another, and they can stack, so if you give her something to replenish her awakening bar quickly, she'll pretty much have double infinite rage for the whole fight, and you can access a lot of her strongest abilities, including ones that give good survivability and protection from one shots/team wipes, you'll mainly want to use Ray as your healer/support, keep the MC and Ray alive, use the deck on the MC to inflict regen/zombie on enemies, and don't play too aggressive, as enemies can counter attack, and you can't counter back, so it's better to play more strategic/defensive.
If you're still struggling, I recommend trying to rearrange your gear and blessings rather than trying to grind, the game says as much in the tutorial, for the hardest stuff, only good strategy can win, and unless your strategy is overwhelming power to an absurd degree through stacking blessings for defense and offense (taking into account the difference between magic and physical damage), you're never going to simply "overpower" the hardest fights in the game.