At the end of Chapter 3, you will fight a big chainsaw zombie. Before entering this fight, you should eliminate all other enemies in the area to make it a lot easier to get the trophy. To perform an easy sneak kill on the Boss, fire flash bolts with your crossbow. A flash bolt can be found in the watch tower on the right side of the area. You can always craft more for two crafting materials per bolt (disable traps to gain enough upgrade materials). Before stunning the Boss, make sure to weaken him. Shoot him twice with explosive bolts, twice with standard harpoon bolts, and twice with the shotgun. You should then be able to perform a sneak attack with the knife to kill him and get the "Knife Beats Chainsaw" trophy. If you did not weaken him enough, just keep crafting flash bolts, shoot them at him, and keep doing sneak attacks until he is dead.