During the Episode1 Story nearing the end (I use that word loosely) you'll reach a Quest called
The Beginning of the End
This can be played on Casual or Hardcore difficulty
After completing the Naverius Ruins you'll encounter a boss. Immediately after finishing said boss you'll be faced with 1 more encounter.
You'll have a full team of AI with you so this will be a very simple fight.
After his defeat the achievement will pop and the mission will end.
There's still a bit of Episode 1 left to go in order to obtain