Imperil doesn't work like that, unfortunatelyTyro has Sentinel Grimoire. I didn't add Power Breakdown because Full Break should cover it, and Fran's Shatterheart is a 610 which reduces ATK and MAG stacking with Full Break. Supposedly you can also blind him but that never worked for me when I tried so I dropped that.
So you think Shout would be better? One reason I was thinking something like Stellar Circle 5 is because it includes Imperil Wind. But I don't know the math well enough to know if Imperil Wind leads to more damage than ATK+50%
There is a scale between -10 and 20, -10 being absorb (-100% dmg), 0 being null, 10 being normal dmg (100% dmg), and 20 being weak (200% dmg)
(edit: AND resist is 5)
Imperil adds 2 to the scale.
This boss is already at 20, so Imperil will do nothing