I had a problem getting this achievement. Hunter Through and Through achievement not working
However I have since gotten it. Here is a summary of what happened and how I fixed it.
My 2 friends and I were level boosting other characters in DLC (against Scraptraps) when I mentioned this achievement. We decide to go for it since you need friends to kill the rare spawns first in order to get the missions, and you need to do the missions for the achievement.
I went and killed all 4 rare spawns while my friends were sorting their banks in Sanctuary (because I knew where they all spawned and could do it fast). The plan was for me to do all 4, they do the missions then go back and kill them again 2 each. I already had 2 of the 4, and I knew Amach was one of them because I did him twice in one day.
I kill the rare spawns. They get the missions and finish them. We didn’t communicate well so they both ended up killing all 4 a second time. However the missions did not show up for me. Fortunately one friend finished sooner than the other, so I noticed this before he finished all his missions. I joined him just before he killed Shiverous and got credit for that mission. I now had 75% of the achievement, so I needed either Fungal Gorger or Voltborn. I had my friend go kill them each 2 times but the missions would never show up. He went to bed, and I made the post.
Skip to the next day, my other friend was on and I asked him to go kill them both for me again. He did, I got both missions and was able to get the achievement.
So the best fix would be to kill them yourself, and when the missions show up for your friends join their game and do the mission together. Had I thought of this, all 3 of us would have gotten the achievements much faster. I only did it for one after thinking about it.
The second best fix would be to wait 24 hours. I’m not 100% sure, but it seems if you do the mission then you can’t kill them again to give someone else the mission for a period of time. If this is how it works, that’s seriously dumb for an achievement.
TL;DR - Kill all 4 yourself, when the missions show up for your friends join them and do the missions together. Or wait 24 hours and have your friends kill them again, the missions should show up for you now.
Hope that helps. It’s a really stupid idea to have an achievement require other players to basically start it for you. I wish Gearbox would avoid this kind of stuff in the future.