In any area where you can see an enemy that isn’t currently hostile, tap down on the d-pad to enter the BD-1 menu, then click in the right stick (RS) to go into visor mode. From here, hover over the enemy until its name pops up (or Scan the enemy if you haven’t already) and then hit the Menu button (the three lines) to open up the enemy’s Tactical Guide entry. Once you do this, the achievement will unlock.