This achievement is awarded by killing the Chainsaw Sadist in Chapter 2 with your knife. Easiest way to achieve this is to combine your explosive and flash bolts. The number of explosive bolts needed will unfortunately vary depending on difficulty and upgrade. I did this on survival and used 3 Explosive bolts and 2 Flash Bolts + Sneak Kills. On casual I believe you can only use 2 explosive bolts and 3 Flash bolts + sneak kills
DO NOT try to weaken the Sadist with the Magnum. A fully upgraded Magnum will one shot the sadist.