I was expecting it to be a bit of a massacre with very few survivors making it through, so I gave the aliens only 1 set of activation tokens, even though they were effectively fighting 4 games simultaneously. I randomised who the aliens attacked based on how much noise and aggravation the survivors were causing.
Turns out, we probably got the balance about right, with the survivors being successful enough that they had to turn on each other as they got near the helicopter. And just enough survivors making it to the helicopter to fill all the seats, with one bloke hanging from the skids.
The survivors also managed to down two flying saucers, even though they were usually damaging them on a d4 vs a d8.