Back 4 Blood

You can get this in the final chapter of Act 6 on any difficulty. In the fight against The Harbinger, in the boss' final form, he will have a health bar segmented into 3 sections. Just bring his health down however you want for the first 2 segments (there are LAWs littered all over the arena to make bringing him down even easier), and then land the final shot on his final health segment with a LAW rocket and the achievement will unlock.

Note : Only the person landing the final blow will get the achievement, so if you're doing it with friends you'll need to replay the final chapter and take turns landing it.