You’ll have to play as Sherry to use the stun rod. Hold to ready the stun rod, and hold to charge it. Release to kill the enemy. An easy place to get this is at the very beginning of Chapter 3. There's plenty of enemies and it's the only weapon available until you collect your guns anyway.
Get within melee range of an enemy, hold , then hold . The initial swing when you start holding will hit the enemy and stagger them, giving you enough time to continue holding and release a charged attack for the kill. Reload checkpoint and repeat until you've unlocked the achievement. Best done on amateur difficulty since one hit usually isn't enough to kill on higher difficulties.