I find that if you can reach def down cap with regular Sniper Shot, there's not much reason to go D'art, especially if you're getting rid of D'arts' HP weapon and swapping to a soul using an Atk weapon and Provisional Forest. D'art will hasten Mammon's assist faster but you can just strengthen your entire team instead and let Mammon handle boosting her assist on her own.

I would honestly just save for Shingen's spear, or you just start on it ahead of time. You'll need Sniper Shot, Tyr and Mammon to hit the cap but that still means 2 free slots for whomever. Probably Jupiter to boost damage and possibly speed up bursting a turn or two. While Marduk could as well, she might be better off as your first sub slot in case someone dies, meaning you can use Raiko instead.

If you wanted to, you can make another party for raids that just utilizes all your damage dealer and damage buffers in hopes that other players debuff.

You can use Andromeda for content in which you have to solo until you are confidant you can clear without her heals (Accessory Quest, Guild Orders, etc).