Blast Radius: Get 3 kills with a single frag grenade.
How to do it:
Start up the mission Second Chance. Progress through the mission until you reach the top of a store (the one with the doughnut). Three enemies will be crowded around a fire, jump down and grab grenades. Position yourself so you can see all 3 targets clearly. Try to aim so it lands in the middle and it should blow up and get all enemies.
Watch the video for more help.
(Made by Brenden3: www.youtube.com/user/teamheadsh0t)