Ok, so there are in reality 2 ways to get this.
Legit - you need to play as genestealers, and attack the Marine from behind. In corridors, this is going to be impossible, although if you kill most of a squad, and have 1 left, you can set this up by having genestealers positioned from both sides of a corridor, and pincer movement him
An easier option is to get a marine in a room, as with 6 movement points, you might be able to get around behind. Also note that if you attack a marine and lose, the Marine will automatically turn to face that direction, so you can set this up by attacking from the side, getting the marine to turn, so you can then attack his back. As ever, you can always save just before your round starts, to replay it if it goes wrong.
Boosting - just set up an MP game and have your opponent turn his marine around, for you to attack. If you get them to do this with their marine sergeant, you will also get;