Allow a soldier in your squad to get knocked out, Make sure it's not one that fails the mission/skirmish. Then run up to the knocked out soldier with another soldier to call in the medic.
When this is done you must finish the mission/skirmish without reloading to a earlier save.
The achievement will now pop the next time you visit the command center in your headquarter. The last part is important to remember, because there isn't really a need to go here after you picked out your squad the first time and didn't meet the requirement at that time.