What About Bob? Achievement in Halo: The Master Chief Collection

This achievement can be earned on any difficulty and on any mission; there is one hidden Ranger Commander Elite in each mission of the game.

You will most likely unlock this naturally over the course of one of your several playthroughs, though it's worth noting that these hidden Ranger Commanders are often gold-plated Elites. Many players have had success unlocking this achievement when killing the several Elites inside the Corvette's bridge during Long Night of Solace or killing Elites during their Lone Wolf run for the "One Final Firefight" achievement. Meanwhile, there are reports that, on ONI: Sword Base, upon returning to the base late-mission, there is a white-plated Elite that, when killed, will unlock the achievement. All in all, just press on through the game and the rest of the achievements and this will more than likely unlock without much effort on your part.