This achievement is for starting up the Photoner Story
Either at the Quest Counter or on the Bridge with Xiera, start episode 5
First mission is "Operation to Recover Persona the Masked" which is just a cutscene
Second mission "Seeking the Hand of Salvation" is the story quest you want to complete.
On Casual difficulty it's a very simple fight with Profound Darkness. A few strokes to the chest and a few to his head and as all the other story missions on casual it only takes a min or two.
Next you'll need to find a small handful of Goblins. No big deal.