When the fight begins, Item Slide the ramps, staying close to the walls as to not fall off the edge. Because of the tremendous speed Link has, he can easily beat Helmaroc King to the top of the ramp and even get behind him.
There are 3 phases to the Helmaroc fight. During the first phase you must climb the tower while Helmaroc is attacking you and breaking the floor. If you are fast enough (which will always be the case if you are item sliding up the ramp), you will never be forced to use the grappling hook to cross the gaps. Once at the top of the tower, Helmaorc will land in front of you. There are several ways to pass him (see below), but if you intend to fight Helmaroc for the heart container for exemple, you will need to hit him with the Skull Hammer to proceed to the next phase.
The 2nd phase is extremely RNG-dependant. Helmaroc can fly around and do charge attacks an indefinite amount of time. When he lands on the floor, he can either peck you, dance, or perform its wind attack. He can only perform one or two of these actions before going back to flying again. The only way to remove Helmaroc's mask is to hit him with the Skull Hammer 4 times, and you can only do so when Helmaroc performs the peck attack. Note that if Helmaroc does the wind attack, you can cancel it early by hitting his mask with anything (using the boomerang being the easiest way to do so).
Once Helmaroc's mask has been removed, the fight enters its 3rd phase. That phase can be one-cycled using a jump-spin attack followed by regular quickspins.
To start, make sure you have 1.5 Hearts or lower for your health (This is important because it slightly changes Link's hitbox for the clip to work). If you failed to get behind Helmaroc King while Item Sliding up the ramp, simply charge another Item Slide into the Helmaroc King and you will simply phase through him after a few seconds since the Helmaroc King is an actor. Now just climb up the floor panel on the very right side, and the Helmaroc King will automatically clip you into the floor (You can alternatavely perform a Roll Clip while climbing up to skip taking damage). From here, just crawl under the arena's inward facing spikes and jump to the ring of collision around the arena. From there, you can just Item Slide the pathway to Ganon's door.
You do not need to have 1.5 hearts in order to clip past the Helmaroc King. To clip, simply hold back relative to Link while climbing up in order to perform a 180 degree turn. The 180 degree turn is fast enough to make the Helmaroc King push you away on the first frame and into the wall, resulting in an odd form of ledge clipping.
After hitting the Helmaroc King with the skull hammer on the ramp, it is possible to Roll Clip onto the platforms that usually close when you activate the second part of the fight. If you successfully roll clip, you'll be halfway into the floor of the main arena. Now you need to crawl under the spikes that surround the arena, and using the skull hammer, jumpslash onto the ring of collision around the arena. When you stand on the ring of the arena for more than about 1 second, the cutscene for the battle teleports you back into the arena. To avoid this, you need to backflip and pull out the leaf to move forward on the ring of collision. Once you make it to the path past the arena, sidehop and backflip until you reach Ganon's door. You can easily open the door by sidehopping into it and then mashing A to open it.
Skip the first hit by gliding with the leaf towards the ledge that's in front of Helmaroc, and by cancelling the leaf at the right time to grab it. After this, take up the rock nearest the spikes on the right side, since this one contains hearts. Make sure you only have one quarter of a heart left. Line up to the spikes and place the rock behind you. Take out a bomb and place it in front of Link, between him and the spikes. Turn around quickly, take up the rock, face the spikes and throw it. If lucky, one or more of the hearts will get on top of the spikes. If so, let the bomb kill you and zombie hover towards it. If you get it, you can jump of the spikes and run up to Ganons door.
Note: The hearts drops for this method are random and may not land on the spikes at all.
Start off by leafing to the ledge, just like the short hover method, to skip hitting Helmaroc. Then head towards the spikes that blocks the path leading to Ganon. You will want to sidle the wall on the left of the spikes to get a consistent position, then drop a bomb and jump on the spikes. Proceed by performing a well-timed sidehop with the right angle, then use the leaf to glide a little bit, and perform a jumpslash right before the bomb explodes. This will boost you up above the spikes, and from there you can head towards Ganon.
Here is the original method:
After the third time you have hit Helmaroc King with the Skull Hammer, lure him into the corner near the spikes. After he pecks at you, and gets his beak stuck in the ground, hit him once more with the skull hammer. This will trigger his 'unmasking' cutscene, which automatically warps you to his head. This clips you past the spikes allowing you to skip his second phase.