Went back and managed to do the carrier (2nd) level last night.

As always, I play on the top difficulty level (Realistic) in god-mode. What's nice is that god-mode keeps track of the number of deaths (you still take damage, when you hit zero hitpoints it increments your death count, then resets your health to full), so I can play through a level and try to minimize my deaths. A good run for me is under half a dozen deaths, which means that I've gotten tactics and timing down enough that I probably could do the level the normal way. If I had the time to go back and waste on being perfectionistic about it.

At the start of the level, you should have your Jungle Falcon (Desert Eagle) pistol (with ~158 rounds), your M4 Carbine (~330 rounds), a P90 MSG, and your machete. The M4 Carbine is probably going to be your main weapon through almost the entire level. While the P90 SMG is nice, it uses up ammo quickly and you'll have trouble finding enough to replace what you use. (OTOH, most of the mercs on the carrier are carrying M4 Carbines, so you'll have plenty of M4 Carbine ammo.)

First obstacle is to get out of the water trap without dying. This involves a bit of luck and fancy shooting. There are 2 mercs and a yellow-suited tech (worker) on the left and a heavily armored merc on the right as you exit the ladder room. Crouch, wait for the first merc to approach (you'll see his shadow), headshot him, and reload immediately. The 2nd merc and the tech will follow soon after. The 3rd merc will approach from the right so be ready for him as well.

Now you should have some decent breathing room. Time to clean up the mess that you made and go stealth again. This is your first "indoor" area in the game. Unlike an outdoor area, the binoc's with their "tagging" feature are basically useless and will require that you adjust tactics. My basic tactics for indoors are:

Walk short: Slow, silent and deadly is my goal. That means I stay crouched at almost all times (unless I'm falling back to a better spot). Also helps to keep me from getting lost because I'm not outrunning my ability to keep track of where I am.

Stop and listen frequently: Mercenaries are a noisy bunch and will give themselves away. Either the heavy tread of their boots on the deck plates or through idle chatter about the heat and the bugs.

Move from cover to cover: Basically, don't stand out in the middle of the room/hallway. Try to always stay behind a piece of machinery, a support beam, or a crate. Pick your next cover and move to it without stopping.

Pick your battles: Evaluate the tactical situation, figure out where you want to fight, and then draw them in for the kill. Same basic tactics that you use when pulling mercs out of the camps in the first level. Have a killzone / chokepoint and then lead them into it.

Distractions are handy: Tossing a stone down a long corridor with side-rooms is a good way to draw out most of the mercs into the corridor. A good stone throw will even have them looking at the far end of the corridor while you fill their backside with lead.

Have a few backup plans: Know where you're going to fallback to if you alert more mercs then you can handle. A tactical retreat is changing the location of the battle to a spot where you have the advantage. A corresponding rule would be to keep track of where you are and what areas you have already sanitized.

Reload ASAP: Nothing worse then starting a firefight and then dying because you ran out of bullets. Switching to a backup weapon is often faster then reloading. Indoors, I also use the M4 Carbine on full-auto mode and control my ammo usage by using short, controlled burts aimed at the upper torso / head. Which should put most mercs down with only 4-6 ammo rounds.

The indoor portion of the carrier isn't all that big. A few corridors, a few rooms, maybe a dozen or two dozen mercs/workers that you'll have to kill. For the most part, they'll come at you in groups of 2-4 at a time, just take it slow and make them come to you.

Once you break out onto the starboard walkway (after you pass through the bomb room, ascend the ammo shaft and exit to daylight), you'll need to move cautiously. There are 4-6 mercs on the starboard deck and one of them is manning a chaingun. Three of them will come investigate any sounds, so stay at the hatch where you exited out onto the walkway and pick those 3 off as they come at you from the stern. A rock is a good way to draw them into your killzone, just toss it short so that it tumbles down the stairs.

The merc on the chaingun is tricky. Bottom of the steps is a good spot where you can see his head, but he can't hit you with the chain gun. Then you can just pick him off using single-shots from the M4.

Creep forward and start looking for snipers up on the upper part of the deck. There's usually one right as you start creeping away from the stairs that is out at the very stern of the ship on the top deck. After you pick him off, you should be able to slide along the wall to your right, staying underneath the cover.

As you approach portside, start looking for a 2nd sniper on a portside platform. He's a pain to take out and extremely difficult to get the jump on. One idea might be to toss rocks back at the chaingun area and see if you can get him looking the wrong way. He seems to be scripted to do a nice swan dive into the ocean when he dies.

Head up the ladder to the 2nd level walkway that wraps back around the stern to the starboard side. Go slow, lean outwards, and take it a corner at a time. You'll encounter 1 (or 2?) guards as you make your way around to the starboard hatch.

Back inside. Use a stone thrown down the corridor to draw out the merc who's talking on the radio to Crow. Enter the radio room and you'll get a cutscene.

Exit the radio room and hang a left. Up ahead is another corridor with a doorway on the right. Take shelter at the bend and toss rocks to draw the next set of mercs into the hallway. Or you can run up, open the door, and wait for them to charge out while you hide behind the pipe.

You'll now be in a room with 2 overturned tables and you'll be able to see daylight above you. You'll probably also hear the helo (either now, or when you ascend the ladder to trigger the checkpoint). I suggesting heading up and triggering the checkpoint, but leave the health and armor alone for later. Because unless you're Rambo, we're not ready to tackle the upper deck yet before we go and soften up the opposition.

Head back down and out to the 2nd floor walkway (starboard side). Start looking for snipers and make your way around to the stern portion of the walkway. The chopper is (usually) going to be hovering down at the waterline on the port side. As soon as you get spotted, fallback along the walkway while attempting to kill the side-door gunner.

The general tactic for any helo is that you need to kill the side-door gunner. Once the side-door gunner is dead, the helo will generally fly away. That's much easier then trying to bring the entire chopper down. Although, it's a lot more fun to blow the helo out of the sky and watch it crash in the middle of a pack of mercs.

Head around to the portside ladder and back down to the lower stern deck. Sprinting from cover to cover, make your way to the very stern of the ship. This will give you the ability to pick off the two snipers on the upper deck. Which is two less that you will have to deal with when you head up top. (I tried tossing a frag grenade up where I kill those two, but didn't have much luck drawing any other mercs to the back half of the deck.)

If you have lots of time, you can try jumping off, swimming to shore and picking even more off. Or going to the portside sandbar, tossing rocks up and shooting anyone that looks over the side.

Me? I headed back to the room with the ladder that opens up onto the deck and went for it. As soon as you pop up there's going to be at least two mercs standing close and waiting to kill you. I'm still working on tactics to take care of them. Maybe by tossing a flashbang grenade up first, or tossing a rock to get them looking the other way. (The flashbang has a bad side-effect of alerting the merc on the chaingun, which makes it impossible to "get the drop" on him.)

My basic strategy for the upper deck is to head for the stern while playing hide-and-seek with the mercs who are towards the bow. I take out the chain gunner as soon as I can get a line of fire on him, then continue falling back to the stern. The rest of the fight is just a straightforward firefight with the remaining mercs. Lots of ducking behind boxes, staying on the move, leaning out and pasting a few shots to keep their heads down.

After santising the upper deck, run around, fill up on ammo / armor / health. Then shoot the chains holding the inflatable outboard boat, jump down, and sail off to meet your next objective.