Outside, you’ll find Ghirahim at the bottom of the pit with Zelda, performing a ritualistic, freakish dance to summon back Demise. He’ll send up massive swarms of enemies to delay you.
The majority of these enemies are plain Bokoblin, which can be dealt with in one slice by now, but they come in swarms, so expect many spin attacks and Skyward Strikes. Your goal is to simply continue down the path – however, I highly recommend actually defeating the enemies on the way, because you will be blocked at certain points, and no one wants hundreds of Bokoblin in their face, Master Sword or not.
The first of these blockades will have a group of weapon-wielding Bokoblin, numbering five or six. Defeat them and continue down. As you do, be careful of the Bomb-wielding Bokoblin – you’ll probably get hurt if you try to hit them with the Bombs in their hands, so wait until AFTER the Bomb is gone, then deal some punishment.
The second blockade features three Moblins. You’ll pretty much be able to use the jump-over-the-shield thing I keep mentioning. Afterwards, continue down. As you do (yes, more enemies. Starting to love that Heart Medal?), you’ll find some more Bokoblin. In addition to the Bomb-wielders, you’ll find some with weapons.
The third blockade features a more-annoying blue Bokoblin. Defeat it and continue along the path. As you do, there will be some Bokoblin Leaders in the group. They can make the swarms impossibly more massive, thanks to their horn calls.
The fourth blockade features two Stalfos. You can take care of them, I’m sure. Just try to use spin attacks so as to deal with other Bokoblins. Further on down the path, you’ll end up fighting all types of Bokoblins, including the Archers, so keep your shield up (no bashing if you have the Hylian Shield ^_^).
At the bottom of the pit, Ghirahim, infuriated with your speed, will futilely summon a last wave of Bokoblin. Defeat them and you’ll soon fight Ghirahim.
Boss: Demon Lord Ghirahim
After defeating Ghirahim, there will be a few story cutscenes. More or less, Demise is unfortunately summoned back, and then challenges you to a battle in another place. He leaves while letting you prepare for the battle.
This is your last chance to do any other preparations – once the battle with Demise will begin, what you bring is all you get. I still highly recommend bringing along the Hylian Shield and two Heart Potion+ (I only used one; the second was for back-up). Of course, in the end, you decide what you bring. Before the battle, heal up by sitting in the chair in the Temple of Hylia, then go to the Bird Statue and save. Proceed to the portal of darkness in the pit below, and confirm with Fi that you are ready to fight for the fate of the worlds above and below the clouds.