Space Hulk: Tactics

Ok, so I don't know if the missions are always in an order (given you can select your route), but I got this in mission 3, where you have to defend the "computer" in a room.

Key here is that you only need 3 men to survive, which makes it easier given your marines are spread out at the start.

What I did was put my assault cannon guy (aka GOD) in the room with the Sergeant, one marine on the right, and 2 at the bottom room.

Key for me was to get the lone marine back to the room, PDQ, then got the bottom 2 marines working on clearing the debris (which sadly made them sacrificial lambs).

Once the rubble was cleared, I was able to set up a defensive position, and just rely on overwatch for all 3 marines (with Assault Cannon Marine in the middle) (see below Screenshot)

Save at the start of every round, just in case something goes horribly unlucky, but in the main, you will be fine, as GOD will just destroy everything in his wake.

Good luck!