Gotta do it with Ymir, Jane, Mikasa and Sasha first. Then MC will start saying (when he wakes up) that it's Christa, Elsa and Mikasa left. Focus on Christa.
Try getting her in the hallway > Ymir stops you. Talk to Ymir, get them booze. Then talk to Christa again and get a new scene. Focus on Elsa next