The Power Bracelet is the item you'll receive on the way to completing the Bottle Grotto dungeon.
Once you have it, you need to complete the Imprisoned Pols Voice and skeletal Staflos puzzle, which will help you reach the dungeon's boss, the Genie.
If you're looking for more assistance, our Zelda: Link's Awakening walkthrough can help.
How to get the Power Bracelet
Once you have defeated the Hinox, the door to the east will open up. You cannot go up the central staircase just yet, so work your way round the outside to the north, where a purple vortex will suck in the Keese - and you if you aren't careful.
Run to the right when the vortex is open until it stops, giving you a chance to run to the chest and open it for the dungeon's Map.
Head north out the room, grab the Rupees from the right chest, and enter the locked door to the left with a Small Key.
Inside is a dark room with... Boos! Swiping these with the sword will only kill them temporarily, so you need to light the two torches with Magic Powder in order to vanquish them properly and spawn a chest with the Power Bracelet, giving you the chance to lift pots and other objects.
How to get the Nightmare Key by defeating Pols Voice first and skeletal Staflos last
With the Power Bracelet (literally) in hand you can explore much more of the dungeon. First port of call - head south back to the room with the staircase going up we couldn't go to before. There is a Stone Owl you can interact with here on the north side with the following message:
"First, imprisoned Pols Voice. Last, the skeletal Staflos."
Move the pots with the Power Bracelet and head up the stairs when ready for a side-scrolling section, using the Piranha Plant platforms (shall we say pipes?) to get you to the other side.
When you head down, you'll come to a room with those aforementioned enemies the Stone Owl was talking about, as well as a Keese. So this is the order you tackle them:
1. Kill the Pols Voice - the rabbit inside the ring of blocks - first. To reach it, move the central right block down, then the block to the left left. Now you have access to the Pols Voice, grab a pot and throw it at it.
2. Kill the Keese in the lower right corner.
3. Kill the skeletal Staflos on the left - as with a Moblin, use your shield to guard against the spear, then go in for the kill.
If you have done the above correctly, then a chest containing the dungeon's Nightmare Key is yours.
How to reach the Nightmare boss room
From the room you got the Nightmare Key, head north to arrive at the right side of the long room at the top of the dungeon, with a locked Small Key door on the right.
Head along the top of the room and hit the crystal to alternate the red and blue blocks so you can leap across to the bottom and carry on going.
Grab a pot and launch it up to hit the crystal to alternate the blocks again, allowing you to access the chest with a Small Key.
Work your way back right across the room, alternating the crystal again as you go, and access the locked door to reach a room with several Pols Voice. Use pots to kill them to spawn a staircase in the corner.
In this side-scrolling section you'll have a couple of platforms - the first on the right will move when you stand on it, but the left won't, instead pulling an angry face. To get it to drop, climb the ladder up and right, grab a pot then stand on it, with the extra weight doing the trick.
Climb up the ladder, then leap onto the central path and enter the Nightmare door when ready.
Inside is the Genie boss fight.
Our Zelda: Link's Awakening walkthrough and guide can help you with the critical path, including the Eagle's Tower and Turtle Rock dungeons. There's plenty to be done on the side, too - including Zelda Link's Awakening Heart Pieces, Zelda Secret Seashells and the Zelda Link's Awakening trading sequence. Finally, there's also the new Zelda Chamber Dungeons feature to unlock and complete.