You really gotta have a screen that says "are you sure you wish to surrender?" because i just barely missed ranger training and instead hit surrender when I was doing awesome. Thats so stupid.
get a gunner and give him ranger training. This may sound stupid but it isn't. It doesn't change his gun but increases his rifle. Do it about 3 times and get him in a bunker. He will easily get 100 XP. Then put a trench inside the bunker to protect him even more.
I lost a game by the last 4 guys escaping all with very little health and it brought my life to exactly 0. I had two 5% life gains for two levels and they never got one life. If they would have, I would have won, but no.