This achievement is story-related.
Back a third time to Bryson’s office. After the scene, no need to go outside, the mods didn’t respawn a second time, sadly. For fun, if you talk enough to James, you can take the Husk head with you on The Normandy. It will be in your room. Now play the next scene however you like, but know you can win 2 Paragon or Renegade points by acting, or not. After the scene, get back on your shuttle and get ready for the last part of this DLC.
As soon as you get down from the Kodiak, it’s christmas, but also one tough fight on Insanity. First, the christmas part. Right in front of you, one level below, is an opened container with lots of goodies outside: a M-55 Argus (you normally already have it, so it will give you 3 levels), a mod (an Assault Rifle Omni-Blade augment), some Star Charts for 2250 creds right of the container, and behind you some Survey Data for 2250 creds on the right side of the « tent » with ammo inside.
Left of this tent is another Survey Data worth 2250, some Pure Eezo Samples worth 2250 creds going towards your objective, and other Pure Eezo Samples worth 2250 creds as well a few meters away towards the ocean (on the left side when you look at Cortez). A little bit farther, near the water, is another easy to miss mod, a Pistol Cranial Trauma System. Regarding the fight, only the 2 Brutes could be a problem. Do not forget to check the destroyed Mech and also destroy every Artifact Sphere you see, as it might have an influence later on*.
After jumping the gap, some Mining Equipment is worth salvaging on the right for 1500 creds. Going towards Cortez is another easy to miss mod, same place as the one before, to the left near the water (Pistol Ultralight Materials).
You should now have every new AR and SMG mod up to level 3 and every new Pistol mod up to 4!
You have a Hydra behind Cortez. Talk to him and launch the next fight. Just Brutes, Husks and Cannibals. Keep your Hydra for the third pick-up when Brutes spawn 2 by 2 to help you in the last push towards the Mech. You have 2250 creds of Xenotechnology to salvage on the left of the Triton ADS before jacking it. Destroy everything, then EXIT your Mech.
*We are going to backtrack now. I am not sure when exactly during the last few battles all of this spawned, but there are new goodies at the beginning of the level. It may be because I destroyed all the Spheres (the last 2 are left of the Mech), or it just may be scripted. So, backtrack to the first container. You now have Skeletal Remains to check and 5 Datapads all around this area. They didn’t give me anything because I’m maxed, but maybe they will give you EXP. I’m pretty sure a lot of people missed this.
Now go back to your Triton ADS and go see Cortez. It's now just a long walk and a long chat until the end of this short DLC. After all the scenes, the Under Pressure achievement will unlock just as you end your chat with Ann Bryson.