Prior to entering Level 7:
1) Up in the mountain. Give the man a whack on the head!
Prior to obtaining the Wand:
2) Basement 1, room 14. You need to drop down from the overworld by pushing a grave to access this section, then use the Lens of Truth in order to get the key.
3) Basement 2, room 03. This is a room (with blood) after the push block puzzle. You need to use the Lens of Truth in order to reveal a key, then slash the key in order to open the invisible treasure chest.
4) Basement 3, room 57. This key is in the SE portion of the third basement. You can enter the SE portion of B3 by bombing the ground from B2.
5) Basement 2, room 35. This bloody room is divided into two portions. Enter from the right side and get the key the same way as in #3.
You will have enough keys to get the Wand.
6) Basement 2, room 46. Simply shoot magic across to trigger the switch and flood the pit; swim across to get the key.
7) Basement 1, room 27 (the very NE of Level 7, B1). Again, just shoot magic at the switch.
8) Basement 3, room 50. This key is in the SW portion of the third basement.
9) Basement 3, room 20. This key is in the NW portion of the third basement. You'll stumble across this area as you're unlocking doors in the NW portion of the second basement. You can't miss it.
You will have enough keys to obtain the 2nd Bracelet. You have to play the Ocarina a few times in the room that has the 2nd Bracelet, BTW.
10) Basement 2, room 21. You have to go to the room E, E, S, S from the entrance of Level 7 in order to get this key.
11) Basement 2, room 16. You have to go under the bloody "X" from the first basement in order to get to this room, first. Then, kill the enemies (which includes the Batrobe as hinted by the wall panel) to make the key appear.
12) Basement 2, room 04. You have to do the block puzzle again!
13) Basement 3, room 06. This key is in the NE portion of the third basement. The easiest way to get here is to go back to the overworld. In the screen above the Inn, slash the bush in the center of the square of bushes in order to fall to B2. It's pretty linear from here. Do not take the portal in B3 before getting the key!
You will then have enough keys to get the Boss Key, which is in the central-eastern section of the second basement. Y'know.