Anti-Virus (Get ADA back online using the Vaccine) - ZOE 1
It looks like this is story-based and unmissable. After ADA is infected with the Virus, you'll get a tip to go to Factory 2 (ADA's "home") to search for an antivirus. This is the same time that the fourth rescue mission becomes available.
Head to the right in Factory 2, toward the white ">>>" symbols on the map. You'll need to go to the far side of a tunnel-like area. Try to avoid battle, since you're in a state where one hit will destroy you. Inside will be a single enemy, which you just need to take care of being careful not to get hit. That nets you the passcode for the anti-virus. Then access the left of the two terminals for the achievement.