This is Not the Greatest Lab in the World… Achievement in Halo: The Master Chief Collection

This achievement can only be earned on the Legendary difficulty. If you don't want to play through the entire mission, you can load The Package at Rally Point Charlie.

After clearing the elongated, final firefight of this mission, there is an opportunity to enter Dr. Halsey's Lab before even activating the entrance button and entering the standard way. As you may have guessed, this can be done by finding and activating a hidden switch within the area, which in turn opens a secret door that leads into the Lab. Be mindful, however, that, after activating this button, as you approach the hidden lab entrance, seven sword-wielding Elites will ambush you.

For a visual aid in locating the aforementioned hidden switch, as well as a general tip on how to deal with this firefight in a more efficient manner by hijacking one of the Wraiths in the area, please refer to the video guide below, courtesy of Halo Completionist.