Them crates man.

Ok, so I've added some more units and a new ability, which is hiding behind crates. Codename M60 can avoid getting any damage (and level progress, duh) by ducking behind these.
When you're hiding behind a crate your health regenerates 4 times faster (although i'll change it for some manual event, like reloading), you don't take damage (but the damn barricade does!) and you can safely reload your weapon. Also, there's no car, barrels and prisoners spawn, so you have to use these crates wisely.

M60 hiding behind a  crate as usual. What a casual!

When your mouse pointer's horizontal position is behind the crate, you duck and the crate gets all the damage you'd get when running. The crates are breakable, so you'll have to take care about these pesky enemies.

This ain't no survival mission, mr. Agent. Get back to rescuing prisoners!

When your mouse pointer is over the "enemies' area" you can actually shoot (no shit Sherlock) and get the normal damage. The same damage as if you were running. You can duck at any moment you want by leading your crosshair back behind the crate. It's pretty logical, why am I even explaining this to you?

RESCUE THE DAMN PRISONERS AGENT M60

My next step is to make the manual healing and change the explosion of limbs for some death animations.