For What About Bob, you must kill one of the game’s golden Ranger-class Elites, referred to as Bobs. These secret enemies spawn at random at certain locations throughout the game and immediately run away from the battle, disappearing after a few seconds. There are many in the game, with seven just in the Winter Contingency mission alone. Load up Winter Contingency - Rally Point Bravo on any difficulty (though I would suggest normal or higher). There are three that can spawn at the first settlement you reach after Rally Point Bravo (the one that you need to go into the building and clear out the Covenant), though only one of the three will spawn during any given playthrough. Starting from the rally point, hop into the transport vehicle and drive straight through the opening in between the buildings at this first encounter. Hop out of the vehicle and quickly go up the hill to the left so that you’re looking down over the rear exit. If any of these three Bobs spawned it will come running out the rear of the opening briefly after your arrival. Drop in behind it and assassinate it for the kill and the achievement.