This is technically missable but this does accumulate through playthroughs so don’t worry if you don’t get it at first. Simply keep some first-aid kits on you; some enemies will do a heavy number on your team mates. They will sometimes call for you but usually the case is they will heal themselves if they have a first-aid kit. So best thing to do is not to give them first-aid kits, and let them fight, this can prove difficult though as if you don’t fight much it could lower trust. I managed to do a fair bit of healing against the mobile missile robot in the tunnel and against the Gorilla bot.