For this achievement you must purchase a combo from the Chop2Shop.Zom and then perform the combo. A Chop2Shop is a shopping kiosk scattered throughout the game's stages where you can buy all sorts of cool things like upgrades, combos, costumes, music, and art! The first Chop2shop location is during the Prologue, and will appear after Juliet's Phone is first introduced and you recieve a phone call from your mom.
The kiosk will have a flapping bird above it with balloons, interact with it by hitting to go shopping. Go under "Awesome Skillz" to buy the Chainsaw Stab combo for 60 Medals then exit out. Now test out the combo by pressing , , to unlock the achievement. You don't actually have to have the combo make contact with a zombie in order for this to unlock.