This I guessing is an older player who doesn't know about the new update that allows you to cap a skill when the boss is dead. Not while its being used.
Yep, basic order of things, Cutscene - Hundred blades boss spawns - You kill boss - Cutscene - You cap Hundred Blades off his body after cutscene and proceed with mission.
I left out the name incase of spoilers but i fear it's too late T-T...