During Mission three: The Good Doctor, one of your main objectives will be to talk to Dr. Hypatia. To get the "Counter-serum" achievement, you must cure her by crafting the anti-serum. Do not kill her or you will not be able to get the achievement. When you talk to her in the morgue, check for a dying patient in the back of the room. He will give you a side quest and the combination to a safe. Grab the counter-serum formula from the safe and craft it. Then, inject it into Dr. Hypatia by performing a non-lethal takedown on her. Complete the mission and you will be aboard the Dreadful Wale (your ship/hub area). Talk to Dr. Hypatia on the ship to get the achievement.