To get this achievement,you need to finish off the two bosses one at the end of chapter 1 and the other at end of chapter 2.I did it in story mode.First get their health way down,almost to the end of the health bar,then,you must finish them off with an action skill which is used by pressing the Willpower icon.I kept thinking the action skill was when you switched between main and sub weapon by pressing that icon which is above the willpower one.Make sure you save enough willpower in the gauge,I had gauge at 3 just to make sure it was enough to kill them.You can look at the skills via menu and select/equip an action one.I used one of the very first available ones.