Follow the tunnel until you reach the end. There's a campsite outside, use it if you need to. As usual I suggest you upgrade your bow. It'll be really useful in a little while.
At the left side of the campsite there's a pillar. Shoot a rope arrow towards the climbable part of the cliff and use the zip line to get to the other side.
Then start making your way to the right. Keep going until you get inside the bunker.
Once you get inside you'll notice that there's a hole at the back of the room. Use that to get inside the bunker.
When you exit the hole you'll see a Solarii in front of you. Silent kill him then hide behind cover.
Start picking off the welders one by one. Make sure to aim for their heads with your bow to keep it quiet.
Climb up the wall on the right suide and get up into the second floor area. Shoot all the welders that you can see from here. Then kill off all of the Solarii below.
After you've killed all of them head down and enter through the metal door at the far left corner of the area.
As soon as you get near the corner the Solarii will send down a barrel of oil and make it explode. Get away from it to avoid getting caught up in the explosion. Then hide behind cover and take them out.
Head up the stairs and more Solarii will come and attack you. Hide behind the metal railing and kill them off. Be careful of the bombs, make sure to dodge away when they throw some at you.
After you've taken them all out head for the platform tright next to you and use the pole on the right side to reach the broken ladder.
Climb up and hide behind the pillar. Aim your bow at the oil barrel and blow it up. The explosion will kill all the Solarii standing beside it.
Use the fallen door to jump towards the wooden crate. Shimmy along the side until you get to the side that you can climb up from.
From the top of the crate jump towards the side of the wall then climb up. Then drop down on the opposite end.
Use the campsite if you need to. Then head over to the cliff and fire a rope arrow at the wall. Climb down then use the zip line on that side to get to the Endurance.