My Sieg isn't leveled up (because he's second only to Avicebron as lowest priority Caster), but I eventually managed it. It was super close though, I had to deplete Amakusa's entire second gauge with a brave chain from Sisigo CE equipped Dantes, and not a single hit was critical. Somehow I managed to just finish him off with the extra attack, but 3000 less damage and I would've eaten an NP and died.