Perform an Assassination against an Elite to survive a fall that would've been fatal. 25 points.
The best spot to earn this achievement is at the beginning of The Pillar of Autumn mission. At the start, move to the mountainside up ahead and look for an Elite standing below. Then, back up, hit LB to trigger Sprint and jump off the cliff and toward the Elite.
The important step here is to hit LB again while in mid-air to disable Sprint, otherwise you won't get the achievement even if you manage to assassinate the Elite. So, hit LB after jumping to switch off Sprint, then hold RB when you reach the Elite to perform the assassination and save yourself from a messy death. This video is from our unofficial game guide. You can view the full guide here