Earn a Triple Kill while Jetpacking in Campaign, Firefight or Matchmaking. 25 points.
You must score a Triple Kill while 'Jetpacking,' which means while boosting. A Triple Kill while falling after boosting with the Jetpack won't unlock this achievement.
This achievement can be unlocked during the campaign (at any point the Jetpack armour ability is available), but doing so requires some luck.
By far the easiest way to earn this achievement is during a Rocket Fight Firefight match. Choose the Jetpack armour ability at the start and move close to one of the first Phantom drop ships that flies in to drop off troops. When the doors open and the Grunts start to pile out, hold LB to boost and be ready to shoot at the Grunts when they land bunched together. This video is from our unofficial game guide. You can view the full guide here