The red mist method took me exactly 60 minutes.
I did it with one other person because 4 player co-op was too laggy, glitchy, and buggy. This includes losing all the gear that I had gained upon traveling from Redfall Commons to Burial Point and having the trust level seemingly reset after the game crashed (more on that in a bit).
Basically, you or your partner stands in the red mist, and the other (standing outside of the red mist) revives the ‘downed’ player over and over until the achievement pops. My partner did absolutely nothing and we both got the achievement.
The green icon in the bottom-left corner displays your trust level, not the number of players that you're in proximity to. I know this because it went to level 3 and 4 despite playing with only one other player. For me the achievement unlocked several minutes into level 3, for my partner it unlocked once they reached level 4. This is because I had already gained trust with that character in other co-op sessions, whereas they were playing co-op for the first time. To summarize, trust appears to be cumulative even if it doesn't show up that way.
It's not bugged nor does it inherently display the trust level incorrectly. If you've never played co-op before, you'll need to get to level 4. If you have played co-op before, there's no telling at which point it will unlock, which is why people think it's bugged. If your game crashes, simply reenter the lobby and keep going. The trust level will appear to reset, but it won't (you'll just lose the ability to know when the achievement will pop).
There's also a trust meter in the menu (hover your character) that's made up of 3 bars. You'll start at 0 and the achievement will unlock at 3, whereas the aforementioned green icon in the bottom-left corner will start at 1 and end at 4. It's confusing, I know.