Both of those encounters are pretty tough. Do the helicopter first because you get good gear from it, unlike the tunnel entrance. You need to scout the helicopter area and pick off at least three of the roaming butchers. After that, I found it best to take up cover with the trees and water to my left. Combination of the roots power with a Molotov works wonders. Spread your guys out so the pyro doesn't get you all, focus on the boss, then try to prevent the robot from healing him. Once you're past this encounter, the tunnel entrance battle is much harder. The two hunters are deadly at range, and you have to focus fire on the shamen and robot. Not an easy combo, especially with your weak squad. Do everything you can to bring down the shamen than the bot, then get up close with some shotgun action and you'll win the day. Don't forget to trim the herd as much as you can and sneak around the back to get the better armor before going after the shamen.
Honestly I'm level 26 now and those two encounters were some of the hardest in the game so far.