The Memorial Mob is a vendor that can be found in several places throughout the game. The first is located near the Outskirts Wall - Gate Path Sculptor's Idol.
Continue forward and grapple onto the roof of the tall building in the next area. You will see a small elevated platform on the left:
Make your way around the base of the platform and look up to find a grapple point:
Grapple up and interact with the vendor in the tent to unlock the achievement: