At this point, I'm going to say that I've (mostly) figured out the Fort and Pier levels (3rd and 4th) for Far Cry.

Fort

I can wipe out the beach patrols and the one manned boat that gets summoned, and I also wipe out the leftmost beach patrol. It's pretty easy to pull these mercenaries in ones and twos, then picking them off at maximum range (75-100m or farther).

Where I have trouble is the bottom of the stairs. It's thick jungle, there are no overlooks that I've found yet, and there's half a dozen mercs that hide in there at the base. I've tried drawing them out with grenades (which has the nasty side-effect of alerting every other merc up top, which isn't really a viable tactic), but the mercs like to stay close to the stairs.

I may have to try drawing them out with stones and the noise from an M4 Assault Rifle. A bit of "bait and fade" to pull them down into a killzone.

Once I clear those, it's pretty easy to work my way up (I like to take the elevator, and I think I get the silenced MP5 SMG in there). The MP5 in single-shot mode, carefully aimed, and fired in 3-round bursts makes for short-work of mercs at 80-120m, but without drawing the entire area down around your ears. And if you fire from cover, it can take the mercs a few minutes to find you, which is longer then their remaining lifespan.

My preferred weapons by the time I hit the part where you get mobbed by the boats/helos is a silenced MP5 SMG, the M4 Carbine, a P90 SMG, and the AW50 Sniper Rifle. I generally toss the sniper rifle in a corner until I've finished using the rocket launcher.

The 2 helos + 2 boats still give me trouble. I tend to just 'god-mode' my way through that part as I haven't found a good set of tactics to survive the assault.

Pier

First order of business: Pick off the 4 mercenaries at the hilltop camp with the hang-gliders. Try to make minimal noise.

Don't use the hang-glider. Unless you like being shot at.

Next, head down the path a bit until you hear the helo approaching. Then, go hide in the thickest foliage you can find. With luck, you won't get spotted and the helo eventually stops searching for you and flies off. (Worked for me once so far.)

I pick off the patrol boat from the top of the cliff (usually takes a clip or two), pick off the 2 mercs over at the chaingun emplacement (far side of the valley), then take care of the 3 mercs down at the waterline. That clears the stage enough that you can go scavange weapons, armor, and health and get ready to tackle the next step.

The mountaintop guard tower is easy. There's a sniping point on the right valley wall about 60-80m away from the tower. You'll have a clear view of them, but they won't see you. Pick them off with the silenced MP5 SMG and it should be easy-peasy.

Softening up the camp is difficult. Sniping at this range will bring a helo down on your head. Unlike the other helo, this one doesn't seem to want to go away if it can't find you. I hid in a bush while it hovered overhead for a few minutes. Finally, I just started unloading my M4 Carbine into its belly until it took enough damage to fly away.

Still working on an overall strategy for tackling the main camp in the bottom of the valley. Tends to be a lot of "fire and fade" and there's plenty of ground cover to lose your pursuers. One strategy is to blow up the large tank at the far end, which will draw the enemy forces away from the near side of the camp.

Make sure you snipe the guard in the tower who is packing a rocket launcher. I was almost ready to take on the main camp (having picked off the outlying sentries) when I spotted him. I ended up backtracking all the way up to the top of the mountain (near the lookout tower) and sniping him from maximum range before making my way back down.

The vehicular ambush is taken care of by parking a jeep in front of the door, then tossing a grenade when the doors open. With luck, you'll blow up both vehicles as well as the mercs who were waiting inside the garage.

The next 7-8 guards are taken care of normally (with stealth and care). Which will put you at the river crossing with the broken down (yes?) truck and a pair of 4WDs. The river crossing is a good spot to snipe the guard in the next lookout tower. (He also packs a rocket launcher and can make your life hellish.)

From the river crossing, head up the hill, make your way to the hilltop guard tower. You should be able to get a good eyeball on the dock area from here. Make sure there's no other mercs remaining anywhere but down at the docks.

Snag the rocket launcher, go prone at the edge of the tower decking, and zoom in. You should have around 7-8 shots, which will be plenty. My tactic is to take advantage of the AI's tendancy to congregate around things that have recently gone 'boom'. Plant a rocket into the middle of the docks (on the decking), then wait 30 sec for all the mercs who are nearby to run over and investigate. Once you think they're all there, drop another rocket into their impromptu meeting spot. I think I managed to kill 8 or 10 from about 3 blasts with the rocket launcher.

Eventually, you'll get the update that it's time to go meet your ride at the docks. Go down, collect your spoils from the dead mercs and meet your ride.