Upon completing the Testing Area, you will reach the boat house. When you enter the boat house, save on the cassette player (this save will act as an anchor to get to other ending as well.) Go up the stairs to see Mia and Zoe tied up. You will fight a giant, mutated Jack Baker after a brief moment with Zoe and Mia. After finally defeating him, you will exit the boat house onto a dock and be given the choice of who to give the serum to. For ending 1, give the serum to Mia. The achievement will pop after completing the game.