NOTE: This achievement can be buggy. If you got the kill but no achievement, check the social club for the achievement before firing again.
This is unlocked by using the orbital cannon that comes as a 900k bonus option when buying or renovating a facility. Only the owner of the facility can use the orbital cannon. Before buying a facility, you should look into the optimal locations for the Doomsday Elite challenges as the first two acts include the drive from the facility to the mission location in the timer.
After getting your own orbital cannon, you should have access to one free shot given to all players a while back. Although it's shown as the manual and automatic shot being free, you only get one shot from either so use the automatic shot and make sure to use it on a friend so they don't suddenly disconnect during the shot (also it sucks to get obliterated by randoms for no reason so please don't be one of those people)
If you don't get the achievement for some reason, check the social club and see if the achievement is marked as unlocked there and if so, the Xbox achievement should unlock on its own eventually. Otherwise, using the orbital cannon again costs 500k for a manual shot and 750k for an automatic one.