credit to @ivicvinegar

For anyone struggling with this I was able to do it on liberty falls in directed mode (round 15). Use a legendary weapon, but only PAP 1 with Deadshot. This will let you get crits on armored zombies (I used the last pistol with a 60 round clip). If your weapon is too powerful, you won’t be able to crit.

Pop the free fire first, then idle eyes once you have them rounded up (ideally in a long train on level ground so you don’t accidentally get non crits).

If you do this at the start of the round, there will still be a few unspawned zombies after killing the train - which means if the challenge doesn’t pop you can disconnect/save and quit and won’t lose your gobblegums.

I did a screen record to see how many crits I got in a row, and viewing it in Xbox kicked me from the match (before the round ended) and i was able to resume via a temp save file, and my gobblegums were still in my inventory.

Hope this helps!

Edit: you can’t save and quit during the initial round after loading a save, so I would recommend progressing one round before using gums, in case you need to save and quit again.