Tom Clancy's The Division
65 Achievements
1,450
75-91h
United We Stand
At lvl 30 start and complete a Co-op Mission without anyone being Downed or Dying.
40
13.13%
How to unlock the United We Stand achievement in Tom Clancy's The Division - Definitive Guide
Once you unlock "On the Level" you can simply get in a group of two or more and replay an earlier mission. You can play it on Normal difficulty, so you should be much stronger than necessary if you choose an early mission. Just take your time and use cover as much as possible, staying close together to be able to share heals.
This appears to be another achievement that can glitch, though it can glitch in both good and bad ways. There are many reports of people getting the achievement even though someone has gone down, as well as many reports of people completing a mission with no one going down and not unlocking it. It will eventually unlock in some manner, so just replay easy missions on Normal until you get it.
You'll be doing a lot of missions at 30 if your goal is to get better gear, so it's entirely possible that this trophy will unlock without really aiming to complete it. You should also note that this trophy is glitched in a good way. The game is not accurate in recording party deaths. If you are in a party where another member is downed, it is still possible to earn the trophy just as long as you aren't downed.
You can also attempt to boost this trophy by joining a coop game with a friend who has gear good enough to solo missions and you stay in cover behind the current objective. If you'd like to be productive, you could also use the First Aid ability to keep your partner healed while they fight enemies.