In the first Pompeii level you will come to a part with a ton of freeze machine, the ones that slow down time, where you have to use them to climb up on falling Gnaar's or just pile up their bodies to climb out. Once you exit the area you will come across the Laser Rifle. Grab the gun and go left, there will be a small area that you can jump down into which will lead you to the Red Light District. It is directly below where you get the Laser Rifle. Walk into the red light district and the achievement will pop.