This is really easy with a friend in co-op
but you can do it in solo easily too
Just shoot a Juggernaut until he kneels down
Then use RT+X on him to finish him
*The achievement will pop for everyone in the game*
this will also pop
Here's a video for this achievement:
You can also do it solo by your base.
Your other survivors will have unlimited ammo
Apparently even if a cpu survivor melee executes him you’ll still get the achievement.
See me kill one at the base in my video @ 22:30