Try to keep him as far back as possible to prevent him from getting extra turns!
Mammon Mammoth starts the fight as Yellow element, which is pushed back by red/orange elements! Then, in his second phase, he becomes indigo element, which is pushed back by yellow/green elements.
Multi-hitting moves push foes back even more!
Hopefully, this helps!
I also have issues with the fight. It seems a touch unbalanced. It's hard but I don't think for the right reasons. Phase one is standard enough for this game's fights but phase 2 is not nearly as easy. With Jumbo Jab and Sleeping with the Fishes doing roughly 30 damage each, it's kinda RNG on whether he hits that sweetspot of hitting Sledge enough where he can soak up hits and heal while the rest of the party can keep spamming him with damage that doesn't get countered. Which would be fine enough if not for how Sleeping with the Fishes works. Gaining an additional hit per party member ko'd means you death spiral super easily. You need to have him at low HP by the time he starts killing people or he just wipes the floor with you. And if he just bullies someone that means the skill is a constant threat. And with no way to heal in this game... I mean I get what the fight is going for. It's a gut check to make sure you can adapt. But at the same time with only so many slots for orbs and needing at least a couple taken up to do damage and keep him pushed back, it's easy to call the fight not exactly fair.
And I will say I can beat it with decent consistency. It's not like the fight is unbeatable or too hard. But it definitely doesn't feel like you have the most control in that fight. You just kinda have to pray the RNG works out. Also putting the final boss immediately after the last fairy bribe with no warning is a bit rough to handle. Especially if you didn't save after beating Mammon
