Story related: Unmissable
When you are nearing the end of lvl 16 you will find
*** Spoiler - click to reveal ***a robot infused version of either Wyatt or Fergus depending on whom you saved at the beggining.
To defeat it you must first open the door to the outside hanger area.
Then throw a Tesla grenade at the Mech. It'll be stunned for only a few seconds so be quick on the next step.
Then run up to it and press the interaction button (X) to activate a cutscene and pop the achievement.