I have to disagree with whoadude.
The first part of chapter 6 was definitely one of the hardest if not the hardest point in the game and both times I got past it was only when I ducked for cover behind the car. The main reason for doing so is for the vantage point. You can see the entire field with relative ease while ducking behind the car. Blind firing the shotgun will easily take care of anyone that rushes you. The only real problem is when they throw grenades at you. You'll either have to move all the way over to the opposite side of the car or sprint towards the building. The problem with hiding behind the building is that it is very difficult to constantly look left and right to see if someone is flanking you from the opposite side. By the time you notice them, you will be dead. I've also noticed that if you are behind the building and an enemy uses the car for cover, he will most likely kill you quick if you don't dispatch him immediately.
As someone before me mentioned, the key to beating this game on FUBAR is getting the best vantage point. In this scenario, it's behind the car. It's not a very good vantage point, but it's the best one in that area.
For the beginning of chapter 13, you will spawn front and center behind some cover and facing a platoon of soldiers (when you die). Most people like to strafe left and right in cover while popping out of briefly to get some quick headshots, but it's way too hectic; especially when they toss grenades your way and you get cut down in a second when you pop out. The way I did it was to grab the 417 rifle (on your left) and the grenades (to your right), toss one into the crowd and then run back behind the tail end of the fuselage of the downed plane (or chopper) so that Adams is positioned to your left and forward of your position. Be sure to also grab the SCAR rifle as you are running back there. With the 417, zoom in and headshot everyone. If they are flanking you, Adams will most likely take care of anyone that approaches on you left. You'll have to keep an eye out to your right, but you'll be able to see them coming from a mile away. It's a lot easier to take out people from this distance because some of them can't even hit you, but you can still see their heads! They key to this method is that you MUST go back to the original barrier (right in front of Adams where you spawn if you fail) BEFORE the armored heavy guy comes out. The reason for this is because if you are anywhere else on the battlefield, the heavy will approach your position and kill you while two or three elite soldiers flank the shit out of you. It is incredibly hard to take them out popping out of cover because the heavy will mow you down in less than a second. For some reason, if you stay in cover on the left side of where you spawn, the heavy and an elite (or two) will stop just shy of where you are hiding. This will allow you to unload on them at point blank range while blind firing. Toss the one grenade you have left while he is far away or when the other soldiers are close to him. Use the SCAR's grenade launcher to soften him up and then finish him off by blind firing the 417. Make absolutely sure there is no on left alive after you down the heavy. The last thing you want to do now is die because of a careless mistake.
For the yacht part, follow "Dat Boi Treezy's" guide and it should be cake. Make absolutely sure that you bring that sniper rifle (from the previous fight) with you. It makes the fight with the heavy a joke. The only thing I did different was grab the shotgun when you head up the stairs of the yacht during the second half of the fight. This will allow you to easily decimate anyone that comes up there with a point-blank blast. Also, order Adams to kill everyone as much as you can. For some reason, he was invincible for me during this fight.
On chapter 14, the part right after your burning hallucination is the hardest in this level. Once you get to the checkpoint at the bunker, you should be fine, but getting there is a pain in the ass. My advice would be to bring a precision, long-range weapon with you and don't ditch it when you pick up the rocket launcher because if you die and respawn, it won't be on the ground where you dropped it. The key to this fight is to take out everyone before you move, even the people in the bunker and on the turret. It's hard, but not impossible. Worse case scenario is that you are stuck with the SCAR and the RPG. Save the RPG rounds for when gunners get on the turret and don't miss. After you pass the ammo refill box about half-way up on the left side, you'll get an extra rocket. Use it on the turret gunner since they are the biggest threat. Remember that when you kill one, another guy will spawn and man the turret, so never think that they are down for good unless everyone in the area is dead. Take heart in knowing that the enemy spawns are limited so once you take out enough or all enemies, you'll be in the clear, or at least clear enough to sprint and hop over the sandbags of the right bunker to trigger a checkpoint.
There is a grenade launcher and a SAW in there. You shouldn't have any problems after this point.