This achievement is unmissable, however can be quite difficult on Ultimate Badass difficulty. Towards the end of the mission you come up against a giant Alien. Get in the loader and head straight to the corner right of the way you came in, knock the giant Alien into the scrap in that corner and he'll get stuck. Just keep spamming the right hit on him and you'll get past the boss within a few tries. Did it first time once I realised he gets stuck. Hope this helps someone.🏆