Did Arwin for the first time the other day. I called BS at first when Mammon basically one-shotted the party. Actually, I first called BS when Arwin lowered Aura's willpower to a Curse I state due to her having done the maid job.
Then I tried it again and realized Mammon only uses that when she has the advantage, and it can be averted with Thunderbolt or a Stasis Bomb. The first few turns of the fight were just stacking money buffs (remember each buff and debuff can have 2 stacks). The slime should just be turned red and then left alone. Arwin has almost no attacks of his own and the slime is basically irrelevant with the +MDEF buff, so the rest of the fight is easy once Mammon is down.