While ascending the Clock Tower in Chapter 12, you will reach a point where there is a switch to activate an elevator (just after the switch that activates spiked boulders). The achievement will unlock if you are able to complete the elevator ride without any enemies getting on the elevator which causes it to stop. If that happens, immediately pause the game and load your last auto-save.
Make sure you have plenty of ammo. Something like an upgraded Riot Gun and TMP makes this a bit easier. If you’re struggling with this achievement, feel free to wait until New Game+ when you have access to the Infinite Rocket Launcher.