This is the second achievement you will unlock and is unmissable.
The achievement will unlock when you complete the main quest - Survive,
this is completed when you reach the town of Goliet.
When you first run into Duras (companion) I recommend accepting his offer of accompanying you to Goliet. He will deal with most enemies on the way, you will still get the XP for the kills but at this early stage in the game most enemies are a threat so any help makes this much easier.