Far Cry Classic

You need to do this at the beginning of the 5th level called "Research". The easiest way to get the achievement is obviously by playing on the easiest difficulty.

The keycard is located in a hut on top of a hill surrounded by a ton of bad guys. But it's actually rather simple to get.

As soon as you start playing hug the wall on the left and basically never go away from it. Use your binoculars every now and then to mark the enemies so you can check your minimap to see where they are looking.

Now just move along the left wall until your on the hill with the hut. You want to get to the back of the hut. Make sure you marked every enemy and take your time for an opening to sneak into the back of the hut and grab the key card. As soon as you grab you'll get the achievement.

Just as an additional tip:

The enemies markers can turn yellow or orange and you can still get the achievement. If they turn red, you will need to restart the checkpoint.

I gunned everyone down using the silenced MP5 and it still worked. I did it from far away so they weren't able to see me. Their markers turned yellow and orange but I simply waited until I had a clear shot and killed them. You can do this for every enemy as long as none of their markers turn red.

Also there is one enemy near the hut who always seems to run for an alarm. Try kill him first or second to stop this happening. If the alarm is sounded, all the other enemies come running and will void the achievement.

I also hugged the left side. Here's how I think it can be done with minimal frustration...

First of all, I can confirm that restarting from a checkpoint is ok. There is a checkpoint just as you approach the objective, which is really handy for dealing with the guards you need to eliminate.

When you start, equip your MP5SD (you apparently start with one if you replay an individual chapter) and your rock (just in case). Scan the area with your binoculars to tag everyone you can and do an occasional re-check as you advance.

Hug the left wall and advance up the canyon. You don't need to go very slow here. There's only one guy who is a problem. He's wandering in the forest near you, perhaps 2/3 of the way to the checkpoint. Just drop him with the MP5 to make sure he doesn't notice you. Nobody will see him go down.

Once you get to the checkpoint, keep going, hugging the wall, until you get to a fairly large, light-colored rock. Stop at this rock and wait for a minute or so for a soldier to walk by and drop him with the MP5. He will walk right past the rock and not see you, so you don't even need to peek. Nobody is going to see that guy die, either.

This is where you need to stop hugging the wall. Peek around the rock. There should be three guys in front of the objective. Between you and the soldiers there will be two rocks, sort of at the edge of a drop-off. Wait until the guys are not looking and move towards those rocks, then go prone. At this point you should be able to pop up and get a good view of the area in front of the objective. Wait until the guys are clustered together and drop them as a group. On easy difficulty they'll go down with just a bullet or three, so you can get all three with one clip. Now you can just grab the objective unopposed. Alternatively, you can try to use your rock to lead one or more of the guys away for a kill, but I did not need to do that.

I'm assuming that the guards always move in the same pattern, obviously, but I'm pretty sure that this is true.

Collect the key card without alerting anybody in research. The achievement/trophy will be voided if the following:

1. The enemies marker turn red.

2. If an enemy near the hut sounds the alarm.

Should this happen to anybody, restart last checkpoint or to the beginning of the mission and try again. If the enemy marker turns yellow or orange, the achievement/trophy is still active. Achievement/trophy pops if anybody was successful.