You'll get this achievement in Level 5: Chapter 3, the name of which is The Huge One.
Now, this guy can actually be a bit of a pain, he can kill you quite easily and due to how the game handles killing him can take a while, I'll try to explain clearly what you need to do without bombarding you with too much information, I've also linked in a video showing you how to beat him, it's worth watching so you can get a sense of what to expect, when to move and where you should move to etc.
Anyway, when the battle starts you want to immediately head over to one of the several pillars around the room and hide behind it, a majority of this boss fight is gonna be spent behind these pillars and without them you just won't stand a chance. Now the boss will shoot out some kind of spinning blades toward you, as you're hiding behind the pillar you'll be completely safe from this, take this time to make sure you have some fire bullets, every 10 (approx) attacks the boss will need to take a short rest, take this chance to pop out from behind the pillar and pop off a few shots at him, you can also throw molotovs at him or even cannisters/explosive bottles and shoot them while they're in the air to damage him.
As you'll see in the video every now and then you'll need to move to dodge a powered up attack of his, it's not that inconvenient though as there are useful items scattered throughout the room, so moving around when you get the chance is actually recommended!
It may take a few attempts but eventually you will take him down and once you do the achievement will unlock!
* Credit to warman854 for the video