In this game marines start with pistols, and must upgrade as they go. To win the marines must make it throught the dark with penalties like minus one to hit.

In the end room the marines should be heavily armoured and equiped so that they can capture then hold it for 3 rounds while they wait for the last transport to take them to Earth. I admit I have never played the game so Im not sure how it will work but please comment

I've created a scenario roughly the same, but I did mine at least 5 rounds of waiting. Nothing can kill the marines in 3 rounds (except maybe the cyberdemon, and he has to be lucky).