If all your scavangers are dead and you've got almost zero scrap, then it's already game over .. Except when abusing the AI shortcoming that Matt pointed out, it's not quite possible to win. Or at least I don't think so. The scavanger are pretty easy targets for the AI, since they can't defend themselves. Not necessarily a bad thing, but when they cost money and you're not really able to build more than 3 soldier before the first wave comes, then most losses will be on the scavangers side. Which means that you can't actually go spend some money on anything else. You need the barracks, you'll need a few soldiers - money runs out, they kill your scavangers (the 3 soldiers aren't up to the invasion force) and game over. Better look for a way to improve that, perhaps let the scavangers defend themselves.

Cheers