A little late to the party. Just now re-rented and running through on FUBAR.
My 2 cents on difficulty. And the damn glitch that came with it.
Hardest missions for me were 6 (beginning) and 14. Didn't think 13 was all too bad.
Mission 6 was just a trial by fire. See what works when you die constantly, and stay away from the car. Stay behind the building. Let them come behind to come kill you, and get their weapon when you kill them. That helps more than you know.
Mission 14, the glitch from hell.
After the first area with the turret, sniper and a slew of enemies, all I can say is stay back and don't tell your help to kill anyone but the turret, are you kill off approx 12 enemies. He will constantly run out and go down, and that's almost a guarantee you will die. I stayed at the original tunnel and picked off almost everyone from there, including the turret.
Part 2 was an hour long "OK, how the hell do I do this"? When I finally did get to the bunker on the right, and took over the turret, I killed everyone off, and started to the wall. Checkpoint. Death. And a respawn back at the beginning of the area, with HALF of the ammo I had when I started. Now it's really WTF do I do now? I'm not going back through part 1 again.
This is when I learned the quicker method. Don't worry about either bunker of turrets. Start out killing the 5 or 6 on left that jump wall. Have help kill 2 near car, and then join him in the bunker. If you take the path to the end, out of sight of left bunker, you are also blocked to right bunker. He will then advance and take a few enemies with him. A this point, jump the wall, sprint to car, then to grenade box. When you reach that, you may have a few to kill, but again, don't worry about the bunkers. Jump the wall, get directly underneath right bunker, and grenade it. Dont forget sniper on the right up high. Ally will jump wall and clean it out. Checkpoint. AGAIN!!! You will then have about another 25 to kill, most of them with the turret. Another checkpoint.
Gt more ammo in the bunker, I believe it was the grenade launcher and a scar. Go to wall to activate the next wave of enemies. Back to the turret and they will come to you. All that's left here is the heavy (Lugo) that comes through the door. 1 grenade and 3 from the MGL took him out.
After this it's pretty self explanatory. Take your time and check the corners.
Achievements for this is a well earned 1000. IMO, harder than any COD that's been made. This is the first game I've played that I wasn't concerned with missions, only checkpoints.