This achievement is for completing the mission called Humanity which is the last mission of the game. While I'll tell you what you'll be up against in each area, it will be up to you on how to handle each area depending on your character, team/solo, and build. This has a really tough fight in it. I was Level 30 and World Tier 13 when I tried this the first time and eventually had to drop it to WT3 (solo player).
Spoilers are ahead - you've been warned.
From camp, you'll head through some tents, and then down an alley where you will be attacked by the standard sets of Perferos, including Alphas. Once you finish this, you'll go over the "hill" and will be required to open a gate on the right. The last Historian side mission is here (Evolution) if you choose to do it now.
From the gate, you'll continue around to the left, through more tents and then you'll have to drop into a small area with several levels where you'll be jumped by a combination of Berserkers, Warriors, Hunters, Bulwarks, Warlords, and Destroyers. Stay on the move and avoid the Javelin Strikes. Once you finish this area, you'll open the next gate.
In this next open area, the characters will talk to each other and begin to fill in what happened on Enoch with Monroy and the Caravel. There are a couple of crates in this area. Eventually, you'll be attacked by a couple of Sciathans (the large birds). Once you finish them off, you'll need to Clear The Debris to proceed to the next area, which is the Caravel itself.
You'll come to the objective marker, which will take you to the bridge. But first, there is a chest behind you and to the right as you face the objective. Once on the bridge, a cutscene will play with Dr. Z and the laptop, you'll find out that Channa's "vision" was nothing more than a video playing in the background on the bridge, and Monroy will make an appearance and explain how the Caravel got to Enoch before you did. Once this is all over, you will begin the final fight of the game - against Yagak, who you fought earlier in the desert at Utargak and where Jakub was killed.
This fight has two phases, and the health bar at the top is split into two segments. The first phase takes place in the hangar, where it's just you and Yagak - there are no adds here. He will attack with Grasping Shock, Overload, Anomaly Rifts, Sand Wyrm, and Chain Lightning. Once this phase is finished, you'll transition via cutscene to the second phase, which is outside and at ground level. There will be an ammo refill here, along with four Warriors. Follow around to the right, then left and go into the open, circular area for the second phase.
This phase is essentially the same as the first, with the exception of periodic Berserker adds, and he'll start using a Hook Chains attack (think Kratos). When the adds come out, use those to heal as necessary since he'll be in a shield bubble anyway. Once this phase is complete, you'll get a cutscene where it appears the drop pods were not able to be called down, but they eventually will be. Tiago will return, and refugees from the warzone with appear.
After all this, the credits will play, and after those finish, you'll get information on how Expeditions work (the achievement pops here), and you'll be put into the Outriders Camp; it's from here you can begin running your Expeditions.
Also, the last side mission of the game, called Pour One Out, becomes available, where you will have to go back to Rift Town to start.