Hiruzen's mystery isnt a prompt mystery. (death reaper seal) It replaces hes Auto atk round. Meaning if it would be hes turn to auto atk (which is usually instantly after u casted a skill and killed him since IA will determine ur prompt activation delayed turn to act has ended) You either need to have a follow up skill activated (that moves before him) or use someone like mabui/gnw sai/onoki to kill him on mystery.