Research Base - Road Leading up to the Base

Peek around the red truck to spot 2 enemies ahead...

...and 3 more men just up the road. There are also 2 more soldiers off to the east. We'll get to them shortly.

Again, my suggested strategy emphasizes chaining stealth kills for maximum XP, but go ahead and use a more confrontational approach if that's how you like to play. There's plenty of cover and room to maneuver.

If you have the Double Shot and True Shot skills, you can probably take out the first two guys with one volley. (Not shown.) If you'd rather play it safer, start by separating them. Return to the spot where you killed the second enemy, grab a bottle, and toss it into the trees on the other side of the road in front of the truck.

This should cause one of the men to investigate, so you can kill him with a silent headshot.

Move out in front of the truck and shoot the second man the same way.

Now use a poison arrow to take out the 3 men clustered around the map of Yamatai. (If you don't have a poison arrow, use distraction to separate them. Even if you have Double or Triple Shot, one of the guys is wearing a helmet, so he's immune to headshots.)

Move across the road into the bushes, near where the first two men were standing.

From here, you can spot the second-to-last enemy standing in the forest ahead. Kill him with a silent headshot as well.

Head forward between the trees and rocks to where he was standing.

Then turn left to spot the last enemy working on one of the trucks.

If you manage to kill him before he notices you...

...this should unlock a nice XP bonus, especially if you have the Finesse skill.