While using the Mantis Cruiser, kill the last enemy with your last crew member on their ship.
I didn't get this just now. I killed the boss with all of my crew aboard (and actually my own ship depowered other than being cloaked.) As I reread it, does it mean you need to have only one living crew member alive and aboard to get it? (Had I known that, I would have gassed my own guys first, except the one....)