After completing the Sukhothai finale you will arrive in Zigurat. This achievement unlocks at the end of the game. Here is a video walkthrough of the whole mission in 2 parts
Your first task is rescuing Gina, there is a grate to the right of the entrance that you can use to sneak in.
There is one big enemy here who can be dispatched without raising the alarm.
I then progressed, sticking to the right hand side of the map, Gina is at the back in the centre but I was able to stealth most the way.
Talk to Gina through the window, there is a key in the room next to her to free her.
Once done you need to sneak into the main building/temple.
This is hard to do with stealth, I got caught near the end but by jumping up the ledge, you can fend off enemies as they climb it.
There will be a lift, 2 guards will get out so sneak to the right to avoid.
Next as you head down, you would need to parkour a bit to restart lift after it is stuck, give it power etc, there is a gap yo the right of the first lift.
Once through the doors, it is fairly simple to sneak into the tent, I distracted a guard by throwing an object.
After cutscenes is the fight with Voss. 2 stages and the first is regular combat, block and attack.
The second part sees you both hanging from a whip and kicking each other, Gina eventually puts out a hand to grab.
Now it's just enjoy the final cutscenes and you are done.