When you're in the ruined building with Aidan.
Go to the right and through the doorway after picking up the glove and screwdriver.
1: On the wall to the north, is a blue crack in the wall. Use the glove with it to get the blue crystal.
2: Now use your love on the side of the machine Merr is working on.
3: Use the screwdriver with the plant at the top of the stairs. Then return to the main room.
4: Use Arthur's cap on the recess in front of the pool with the square cubes.
Your achievement should now unlock. I wrote this guide from memory, so let me know if something is amiss.