Research Base - Area with Rock Outcrops and Platforms in Trees

After swinging across the branches into this area, you end up on an elevated wooden platform.

If you wait quietly, the men below will soon run out of things to complain about and wander off in separate directions. You can then take out the 3 nearest enemies one at a time.

Use silent headshots for the one in the middle with the ski mask...

...and the one on the left.

For the guy on the right, you can either use a headshot or jump over to the broken branch above him...

...and then, when you see the Melee icon above his head, press Y/F to drop down on top of him.

Next, head to the north side of this area, hiding in bushes along the way...

...until you can scramble up onto a flat rock about halfway along the north edge of the clearing.

You should be able to see the last 3 enemies from here, and if you stay back from the edge, they won't notice you. (There are plenty of other places you can hide, so by all means experiment, but this worked pretty consistently for me.)

Wait for the man on the left to move around the side of the rock where you're standing. If he takes too long, you can shoot an arrow into the trees to draw his attention. When he's far enough from the others not to be noticed, take him down with a headshot or melee attack.

You should be able to handle the 2 remaining enemies, but if you have the Finesse skill and want the XP bonus for chaining a sequence of stealth kills, stay hidden and use a poison arrow to kill them both at once.

Et voilà!