Shadow of the Tomb Raider

This side mission is unavailable until you complete the Mission of San Juan main mission. After that, you acquire this side mission by speaking with Jonah. Prior to the completion of the campaign, he will be in the Skull Cave in the Hidden City. If you complete the campaign without acquiring the side mission from him, then go to the Mission of San Juan in your NG+ playthrough. Find Jonah inside the mission gate and talk to him to acquire the mission.

I don't believe there are any special preparations needed, but just so you know what to expect, you will be fighting modern Trinity, and most of it will be loud.

You'll be going to the Ruined Tower base camp (where the turkeys are) at the Mission of San Juan. Climb the tower and tether over to the little raised part where a monolith is. This is where you begin the side mission.

Find the generator, then find the backlit opening Lara has to squeeze through. (I call these openings "boob squeezes" because I'm usually looking at Lara's boobs while she's squeezing herself through them.) Go through the door where the crab's pincers (aka claws) are pointing up. Defeat the small Trinity force, put Zipacna's crab in your back pocket, and leave the area.

Platform your way into Zipacna's temple. Solve the challenge tomb. This will unlock:


Next, proceed to the Trinity ambush. This is a basic shooting gallery-style combat. Lara has to kill over a dozen enemies, but because this is a video game, they come at her a managable few at a time, rather than an overwhelming all at once. Note that at one point during the fight, the sides of your battle area will fall away and you will become one with the lava if you're out there, so stay in the middle-ish part of your position. When the enemies are all dead, zip across and give Zipacna his special favorite back-pocket crab.

Lara is back at the ruined tower. Return to Jonah at the mission gate and talk to him again. Your acheivement will unlock here, and you will receive the Whispering Scourge - Pandemic shotgun and Reptile Hide - Leviathan outfit, which you need to unlock:

Please let me know in the comments if I left out something important. That would do more good for the community than an uncommented downvote would.