From Halopedia, the Halo wiki
Killed Juggernaut is a medal awarded in Halo 3 and Halo Reach Multiplayer for killing the Juggernaut in a game of Juggernaut. It is represented by an octagon with a green MJOLNIR helmet with a target overlay with a border of 16 outward-pointing green spikes. The easiest way to earn this Medal is if the following scenario occurs: the person next to you kills the Juggernaut. Start shooting them, but don't kill them until they turn white, otherwise it will count as a betrayal.