In Chapter 6, turn off all the Sun Lamps, then travel back to where the cargo lift is - the “H01” platform. Use a smaller lift (not the cargo lift) to travel west, climb into a room and activate a console to the west to turn off the fans and activate a timer. You’ll need to head into the nearby room, kill a Queen Grashtrike and two Sentry Rays before time expires, then activate another console to complete the maintenance routine and open the way to get behind the fans. If you fail, don’t fret - return to the room to the south and activate the terminal there to get another minute, and this time around you’ll have to defeat three Grashtrikes. However you manage it, you’ll be able to grab the Chocobo & Moogle Materia from behind the fan, earning the Cleanup Crew trophy in the process.