Is there anyone out there who has completed this level?

Having completed all levels, but now I'm disgusted. I think it is impossible to complete this level. In which case it should not be offered. Entering "Research" it does not matter which route one takes - overland, swimmimg, hugging the mountain or a combination of both - having successfully removing countless mercenaries, one is suddenly disposed of from god knows where by a single mystery shot.

I know I have to reach the hut to collect the pass-key. Is there a short-cut that I am missing?

TIA for any help.

No shortcut. Here's how I do it.

Go straight away to the top of the nearest bluff and use the binoculars to tag all the mercs. Make sure you get the ones to the left side that are coming around the lagoon towards you. If you have the sniper rifle, lay on the beach and start to pick them off. After a minute some will come at you through the rock arch to your left. Get them with another gun so as not to waste sniper ammo. There is plenty of MP5/P90 and M4 ammo on this level but no sniper ammo. When the sniper is empty, discard it and pick up another type gun.
When the chopper comes, tag those guys with the binocs as well and do a fresh sweep with the binoculars every so often to pick up any mercs that were hiding. Once you have cleared all the red dots its safe to go to the hut. But there is one more guy in the cave.

Research is one of my favorite levels... especially if I manage to bring along a loaded sniper rifle as I enter it.

I like to take a long swim to the right and climb out of the water and about half way up on the rock outcropping that frames the right side of the map. If you swim too far you will be killed by the death copter... You will know the rock outcropping when you see it.

Once I climb into postion (up the rock as far as possible) and go prone, I binoc the area and tag all the mercs I can see. I like to torment the mercs on the outer island first. After clearing the outer island I make a quick swim to it and snag the vest/health and ammo available. At this point I either stay on the island and battle it out with the few mercs that are working their way out to me, or I can swim back to my rock perch and pick them off from a distance.

Once I have eliminated all ememies within view, I begin to make my way to the hut traveling along the right side of the map past the cave entrance. As you mentioned you will need the key card in the hut so I work my way up there and binoc a lot to tag and pick off any random mercs that may still be alive.

I usually jump on the RPG turret and have some fun blowing stuff up before I eventually head back to the cave and nail the last merc who is getting a desperate call from Crowe on his radio. Hope that helps...Good luck!

I didn't quite get it Coops the word "bluff" threw me so I had to replay the level. But, that's pretty much how I played that level as well. There is a 10 pack of sniper ammo on the small island close to where the worker was fixing the boat by the way .

Spoiler
Anyway, at the start of the level, I move a little further on the island away from the arched rock, peek over the hill to tag the mercs on the small island and around beach below the target "hut". Then I snipe as many as possible with only one clip (5 shots) on the small island. Then quickly move back to basically the same spot the level starts at, lie down and shoot the mercs through the arched rock as they come running to get me.

Then run up and go to the left of the arch rock and forward carefully. When the area is cleared back down to pick up the ammo by the arch rock. Then back up and follow the mountain wall and move towards the hut. Enter through the back, pick up the goodies, exit through front...check point.

Sometimes there's a couple mercs that hide (or get stuck) when they were going towards the arched rock to get me. They're by rocks below the "hut" by the first bridge going to the small island or they decided not to take the trail to arch rock and they're stuck in the water on the way to the arch rock.

At any rate....use you binos and radar!!! You'll find them

Funny I cheat the whole game first to learn where to go. If you cheat with ammo you could snipe the whole place. Otherwise there is only one route on this level. I like to go high up above on the other side of the bay. When you hide behind a bush they will come at you without shooting. The hut up there has health and armor. There's only 22 mercenaries on this one. Take your time, don't just come out blazing