"BLAM" Said The Lady! Achievement in Halo: The Master Chief Collection

This achievement can be obtained at the beginning of the aforementioned mission. When you reach the open courtyard fight against Brutes, Jackals, and Grunts, you’ll need to locate the UNSC weapon cache on the left-hand side of the courtyard; it’s tucked away underneath a small awning with red lights. Once you’ve picked up the Rocket Launcher here, proceed to the objective and wait until you reach the section where Virgil need to pry open the elevator doors. This will take a while, so use the time to swap out Dare’s Pistol with the Rocket Launcher you picked up earlier.