Well, the point is, this specific content. Outside of Lust, stacking Dartagnan with Amaterasu is madness indeed. Unless you're looking for heals, there probably isn't a better Soul against Lust? And using heals in Union fights tends to end badly, since the high-leveled Ultimate fights will oneshot you regardless. I guess Joan is an option by giving you a couple of extra turns before the next Overdrive oneshots you.

Assuming that the success rate of debuffs is 30% and you're running triple debuffs (Sniper Shot, Ambush, Amaterasu) you're likely to get... about 1 (def) debuff up? Let's have a quick look, 34% of the time you'll land none, and... 9+6... 15% of the time you'll land both? So 51% of the time you'll land one, I guess? Somebody could do the proper maths on that one.

I guess running Ambush is still better than running Arthur's atk buff. Goes to show just how damn bad buffs are in this game compared to debuffs.