If you get all the Star Metal and raise Trademond to level 10, Verdeaux should be around level 6, iirc, at which point you only need one Power Core from the Clockwork Forest to get it to level 8. Alternatively, there are 4 total Power Cores, so if you're at level 4 you could just use two.
If you're out-speeding the enemies, blind + tactical advantage will give you a pretty good chance to avoid taking damage from any particular attack, and then the goal is to take as few attacks as possible.
My strategy for that fight, in full (obviously my Aura & Slime could have wildly different stats from yours, and John may have different skills, so I can't promise it'll be helpful):
- Activate all the debuffs throughout the domain
- Party is Aura (Lightning Sword II & Auto: Tailwind on), Earth Slime (Auto: Brittle II on), John, Paul
- Turn 1: Aura casts Heat Up; Slime throws a Flash Bomb+; John casts Fire on Possession; Paul attacks Resentment
- John and Paul have tactical advantage, so the odds of taking a big hit here from one of the Greed Demons are roughly 3/8 per attack; i.e., there's a 25/64, or roughly 40%, chance of taking no damage, ignoring effects of Luck
- Turn 2: Aura casts Heat Up; John & Paul Pierce the Wrath Demon; Slime morphs Fire
- This turn you should only face one Furorem attack from the Wrath Demon, which won't be too bad if it hits Slime
- Greed Demon Cupiditas should only deal 1 damage, so all that matters is they don't get tactical advantage from targeting Slime
- Turn 3: Aura casts Light on the Wrath Demon; John casts Fire on Possession; Paul Pierces the Wrath Demon; Slime casts Fire on Greed Demon A
- Hopefully the Wrath Demon goes down here, preventing the 2x Furorem attack from going off, and the Greed Demons should be busy casting Avaritia as long as they didn't hit Slime with Cupiditas; if they did, they'll Slash and it's probably curtains
- Turn 4: Aura Pierces Greed Demon A; Slime casts Fire on Greed Demon B if the Wrath Demon is alive, on Greed Demon A if Aura doesn’t have tactical advantage, otherwise Possession; John Pierces Greed Demon B; Paul Pierces the Wrath Demon if it’s alive, otherwise Greed Demon B
- If the Wrath Demon died last turn, then both Greed Demons might go down here, although Aura acts after them so they'll still get at least one attack off
- Turn 5: If Greed Demon A is alive, Paul Pierces it and Slime casts Fire on it; if Possession is alive, John casts Fire on it, and Slime does too if Greed Demon A is dead; Aura guards
In total, you can face as few as four attacks here, and some of them can hit non-lethally. But even then the odds of making it through the fight are admittedly not great.
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I think the reason that there aren't guides for Richard's domain is just b/c it was released relatively recently.
However, I actually kind of agree with your overall point about Richard's domain. It feels like the least fun part of the game to me, because the difficulty spike is pretty insane, and there's no rewards for the fights. I did manage to get through the Greed and Wrath floors without using the SK (though if you think the Greed floor boss fight is bad, the Wrath floor is... oof... Hi-Sloth Demon Acedia is unreal), but I had to throw in the towel on the Sloth floor, and it's not looking good for the Envy floor, either. Obviously I could spend more time building up Aura & Slime's stats (though that seems like a slow process, and I'm aiming for a minimum days possible sort of run), but that wouldn't help with the fact that it's really hard to get value out of the various party members at times. E.g., since he doesn't have Pierce, Desmond mostly can't do damage to the threatening enemies, even with help from Brittle or Heat Up (at least, not without spamming bombs, and while clever use of bombs here or there is fun, I don't relish the idea of having to chuck tons of Blessed Bomb+ during every fight... though I did end up using 5 of them in the Wrath floor fight). And as you note, John's supposed tankiness ends up amounting to very little - even if I'd taught him Defensive Stance, it wouldn't help against Acedia.