This achieve seems daunting at first, but the short story to this achieve is that it is NOT necessary to exit the map/win the mission in order to pop the achieve; rather, all you must accomplish is to have all 7 marines active (alive) at the same time. In other words, as soon as you wake the last of the 5 unconscious marines, provided your original two are still active, the achieve will pop.

Use the psyker's Force Barrier to block the 3-way spawn location near map-left, and put it as close to the spawn points as you can; the more spaces you block off, the less stealers can mob up that area. As you move the psyker around, you may want to use the Barrier before he moves, since its range is limited, and you DO NOT want the stealers to get past the intersection that allows them to reach the next hallway up If they do, they'll eventually overrun your marines, and you'll have too many angles to cover without enough big guns/powerful melee units to cover them. If stealers spawn from the point above that 3-way, then go ahead and block the intersection and bottle them all up there.

The key is just to buy time; remember: if you only want the achieve, and don't care about completing the mission in the same run as you pop the achi, then all you have to do is have 7 active marines.

If you do want to complete the scenario, it should be pretty simple from here; judicious use of overwatch, flamer to block stealer movements, use psyker where/when needed, and run off the map.

Happy gaming.