The start of this Chapter is the tricky part as you're without Adams and Lugo for the start of it. You'll start the Chapter behind cover and with a Desert Eagle in hand but you'll be surrounded by a few enemies. You're going to want to make sure that make no mistakes with your DE because it's a one hit kill but you're limited to only 5 bullets.
You'll also want to take note, that to your right, but up the area a little is a grenade box and a shotgun. You're going to want to run and grab these. At the start I ran to these, grabbed them, ran to the other end and killed the guys that'll be to your left with a grenade. Once done, I ran back to the car at the start, and defended until the squad turned up.
Either way, aim true and watch your sides! Remember to use the grenades wisely to clear a few enemies in one go! The video below may help.
Chapter 13; Part 1:
For me, and I think for most, Chapter 13 will be a little tough! Here's the tactic I used.
Once Adams, is against the helicopter, take cover against the right metal plate in front of him. If you need more grenades, pick them up while still in cover. Then strafe left to the edge and pick up the 417. Now swap to the left cover. Doing so, the AI should move about a bit. Note, if you need ammo, pick it up here.
When that happen, throw a grenade to the right section lot and then a quick second to the left lot. Aim for the middle, so the dust cloud gets a lot of them. Once done, QUICKLY get out of cover, run left towards the water. First pick up the grenades next to the cover you were up against - near a sunken object - then run towards the water but back a little.
You should be up against a sunken boat and level to the crashed helicopter to your right and looking down the map at where all the enemies are spawning. Being at the back of the boat, you'll also be able to leap over into the middle of it for extra cover, but you shouldn't really need to do this.
From here, no one from the right will shoot you, and you can then look up ahead with the 417 scope and kill the AI when they spawn. So take your time and kill off the enemies one by one. Only 2 guys should run up close to you, so deal with them quickly, but hopefully Adams will get them.
Once you've cleared the area and the JN comes. Try and kill the 2 guys first, then run to the rubble cover behind you. With all your might, grenades and a little help from Adams, you should have no trouble taking him down!
Chapter 13; Part 2:
This is the part where you get ambushed by the big beached ship. The video below shows you a good tactic at the start, so I'll leave that to do the job. Just remember to take your time and to take out the JN first!
This is a whole Chapter video, but I feel the PT 1 tactic I've mentioned above, may work a little better than what is show in the video below.