UPDATED FOR MORE FIXES AND DETAILS FOR OVERALL CLARIFICATION
UPDATES TO BOTH THE SOLUTION AND THE COMMENT
For those of you who are struggling to get this nailed down, we figured it out! lol It took trial and error but we got it!! (This will get the missions to spawn every time!)
You and a friend BOTH have to be logged in at the same time.
You have to stay at the lodge while a friend slays these enemies or the kill missions will not pop. As your friend slays the enemies the kill missions will appear on the wall behind the DJ as soon as it dies. For that reason, it's recommended you stay on the second floor so you can communicate with whoever is doing the run that you received the mission, but it is not necessary.
You BOTH have to friend each other, Whoever is doing the run can verify they added you by looking at the lodge map and hitting right on the DPad twice to see who on their friends' list is loaded into the lodge, they should see you waiting in the lodge on the map (unless you are appearing offline). You should also be able to see them on the map where they have loaded into.
After your friend has slain all 4 monsters and you have all 4 missions on your wall have your friend join your game at the lodge (that is if your friend is also trying to get this achievement).
Accept all the kill missions and go kill everything. (Make sure you have the correct missions selected as you go through your kills, just to be safe.)
Once all needed kill challenges are completed the achievement will pop for you (and your friend if they joined).
If you would like help with this, feel free to message me or my better half and we would be glad to help!
LilChicka1017 or HAHAAHAHAAHAAHA
(Please see comment on this solution for more info on us helping!)
I do recommend to whoever is doing a run to do Amach first. Simply because he is very easy to kill and quick to get to. That way if it doesn't pop immediately for the person waiting at the lodge. A Quit and Reload won't really be a hassle while you revisit the steps to figure out what went wrong. (It's easy to forget to add someone as a friend or maybe they already have that kill mission in their log and didn't even realize it, it happens.)
After you kill Amach make sure to get that verification that it popped before moving on. It's just a time saver trust me =P But if the first one works, you shouldn't have any issues with the rest.
COMMON PROBLEMS AND FIXES FOR THEM:
*Monsters/Target not spawning
If a monster isn't spawning for you, you have to quit the game and reload. Fast travel will NOT reset their spawns. Each monster only spawns once per game load, that's what makes them rare!
*I loaded in from the main menu (fresh game load) into Negual Neshai but Shiverous won't Spawn.
This is rare but it happens for some reason, after loading in and going to trigger his spawn if nothing happens just jump off the cliff to kill yourself and he will now spawn.
*A friend has killed a rare spawn but the mission didn't post to you.
SOLUTION 1: Remember you BOTH have to add each other as friends, double-check that this was not missed by someone. If everyone is friended see solution 2.
SOLUTION 2: Double check that you don't have the mission in your logbook already. Sometimes we pick things up and forget. It happens! If it is in your logbook already it will not spawn on the wall behind the DJ. If you do not have the mission in your logbook see solution 3.
SOLUTION 3: In this instance make sure your friend didn't have the kill mission. If your friend kills a target with the kill mission for that target accepted in their logbook it WILL NOT provide a kill mission pop up at the lodge for you. Your friend has to kill that target to complete the mission then reload the game to kill them again so the mission will pop up for you.
I will try to continue to update this If we come across more fixes or bugs in the run...but we've done this many, many times now and I think (and hope) we've found em all! lol