Firefly Achievement in Halo: The Master Chief Collection

During Data Hive, you will eventually reach a corridor with Active Camo’d Brutes. Upon killing these Brutes, they will usually drop 1-2 flame grenades. Make sure to pick these grenades up and proceed through the mission as normal. You’ll will eventually encounter a decent amount of Drones, be it in subsequent corridors along the sublevel or later in the mission when you pass through the Hives with Dare. Despite your preference on where to obtain this achievement, simply wait until a Drone has landed on the ground, dropping their defense, and then lob a flame grenade at the creature. The blast will kill the Drone almost instantaneously and the achievement will be yours.