For this achievement you have to defeat all four bosses that are found before QuAsSy. They are: FRANK-Y, B-DSS Knight, Herbie and C.O.P GORE. This achievement requires multiple playthroughs as you cannot encounter them all in one run. You can defeat them in any order and once the fourth one has been killed, you will get the achievement. I have not described how to kill each of them here as there are already guides on how to do that.