Dartagnan kinda overlaps with Amaterasu, don't you think?
That said, that broken good SSR weapon Dartagnan has might be worth it.

Anyway, if you want debuffs, then bring Ambush instead of Arthur's atk buff. It's wayyyy more damage output. And because Ambush is the same frame as Nergal, Baphomet becomes better.

But if you mean for this Union event... ... well, chances are you just need to forget about debuffs.