This is a very simple achievement to obtain, but requires some progression through the game to get to the "El Stomacho" levels. They are essentially levels that take place entirely within El Stomacho's, well, stomach. The first two levels of this are level 26 and 27. However, the achievement is slightly glitched and will require you to complete the next set of El Stomacho levels, which are levels 32 and 36. After completing these four El Stomacho levels, the achievement should unlock. If having trouble completing the level(s), please refer to the video walkthroughs below for visual assistance.
Note: It has been reported that this achievement can actually glitch early as well, as early on as level 25. Thanks Srfsmurf for pointing this out.