A quick way to unlock this achievement if you want to do this in single player is to load up Leon's campaign, Chapter 1-1 (Visitor's Room) on Amateur. Make sure you select Helena as your character as she's the only one who has the Hydra weapon. Play through until you have to enter the elevator. This part should only take 5 - 10 minutes Dependant on how fast you're going, may seem like a lot but once you get to the area needed it will only take 5 minutes (if that) to actually get the achievement done and dusted!
using the default weapon you're given (pistol) as this means your Hydra will still have 3 bullets and it wont be wasting precious ammo.
As soon as the cutscene is over and zombies start pouring in switch weapons and press LT + RT together and Helena will quick kill whatever is front of her. Rinse and repeat using your ammo and once you've spent them all just reload the checkpoint. You will begin from the point the elevator doors open.