For this achievement, you have to defeat the boss of the Sewer area: Herbie the Scavenger Queen. The boss symbol on the map in the Sewer indicates this boss will be fought if you go in there. As it is with every boss, you should prepare Heart beforehand as once you go in, you can't come out.
This is arguably the easiest boss in the game due to how easy she is to avoid and kill. All of Herbie's attacks can be avoided just by staying away from her. You fight in a huge boss room and so it isn't difficult to keep your distance. The one attack you should look out for, however, is one where she circles you, trapping you in with her body and summoning enemies to attack you. Herbie's body will damage you, but it will also damage the enemies. You can use this to your advantage to dispatch them with relative ease. In order to do damage to Herbie, you will have to wait until she is above ground and she throws herself across the room. You must then attack the explosives that are inside her by entering her body at the back of the segments. Each segment has one explosive which has a timer started by one attack; so attack it then retreat so you don't get damaged yourself.
Once all of the explosives have been detonated, the boss will be defeated and you will be rewarded with this achievement. You can then enter the pipe from where she came to progress to the Space Elevator.