Tried KoTH, tried CTF, tried BTB. It says “Kill every member of the opposing team at least once in a match” with 0/5 as the number to hit. I’ve killed the entire team twice in some cases but it isn’t unlocking.
Any advice?
Bump
I have the same problem. I got 1/5 somehow but now it won’t count anymore…
Also having this problem, getting kinda frustrating
Oh, I thought it meant instantaneously, like the highly coveted Extermination medal. I should have completed it then. 
I am also having this problem.
I am also at “1/5”. Are we supposed to get an Extermination or just kill each member once?
I figured it out! Sorry the answer came so late, but I spent the day trying to figure out how to complete this challenge. You DO NOT need to get the extermination medal to complete the challenge, you only need to kill each player once. Here are the tricky parts: nobody on the opposing team can quit during map voting lobby or the system acts as if the player was there, but you never killed him, and I only found success when doing playlists that weren’t slayer-based (i.e. oddball, CtF, KotH.) Hope this helps you all out there.
this was super easy just played infinity slayer and got it within 5 games lol
Easy peasy. I already have that Mastered.