
Make sure to stack up on health potions and the likes before going after Ghirahim. Once all the preparations are done, go through the Gate of Time. You'll face a gauntlet-like event. Most of it is pretty straightfoward and the canon-fodder enemies aren't tough. One thing of note is the blue Hobogoblin, who's actualy a much more sophisticated enemy than the others. Once you reach the bottom, the fight starts.
At first the Demon Lord is invulnerable, but even though he focuses on defense, he can still dish out quite a punch, so be careful. As you won't be able to damage him you need to find alternate means - push him to the edge of the platform and throw him down. Just as he falls, use a fatal blow to drop down on him. He will now occasionally stretch out his hands trying to catch your blade. Cut between them to avoid that from happening, but not that each time you hit him he will change the angle, so be careful.This is now you last chance to stock up on items and repair / upgrade your stuff. Once all the preparations are complete, head through the portal to face Demise.
Demise might be a simple fight, but it's by no means an easy one. The trick to winning are shield parries. Parry his first attack, slice at him once or twice. He will try to attack you, parry again. If you get your timing right you should be able to parry each of his attacks while getting a few hits in every time. After a few chain parries Demise will fall to his knees, allowing you to slice at him.Congratulations, you have beaten The Legend of Zelda: Skyward Sword!