After obtaining the Mogma Mitts, deathwarp to the first statue and save. Activate BiT and BiTWarp using the Upper Academy statue. The room you end up in will be partially loaded, and you must reach the door across the broken bridge to reload it.
To reach the other side, first do a manual jump to get on the right wall. To get around the pillar, you will do a forward jump followed by a jump slash. This angle is precise. To get the correct angle, while holding Z, slowly angle the control stick until Link's feet are in-place walking forward, and Link should not be drifting to the left at all. Because of the round shape of the pillar, the forward jump will push Link to the right and the jump slash will bring him back to the left.
Once on the other side, go through the door into a completely unloaded room, and then return back through the door again. The bridge room will now be fully loaded; move to the left side of the bridge and jump down to the passageway in the statue's mouth. From there you can play through normally to the Boss Key.
In the two level room with the water plants and frog statue, it is possible to avoid going to the second level. Get a water plant on your sword, run to the area below the frog statue, and do a front hop jumpslash. The water plant should shatter on the frog statue, triggering it and lowering the fire wall.
When you fight the two Magmanos before the Mogma Mitts, there is a consistent strategy to kill both with a single water plant. At the beginning of the fight, run straight across the room to the platform and grab the water plant at the top. Then drop down and stand next to the platform. Right before the Magmanos attack, take a couple steps backwards towards the wall. One Magmanos should raise the platform while the second should emerge right next to the first. You can then hit both with the water plant and destroy them with spin attacks.
The strategy here is the same as in the first Ghirahim fight, but can be done in three cycles as follows:
Vertical spin attack for the last one causes him backflip faster.
Ghirahim has multiple attacks during this phase, and he attacks in an unpredictable pattern.
Ghirahim will attack you with both swords with warning. You can shield bash and horizontal slash to cancel it. It doesn't hit but faster than backflipping to dodge and he continues Leaping Strike sometimes.
Ghirahim will do nothing at first but suddenly attacks without warning. He is vulnerable right before attacking, so if you can recognize this attack coming you can preempt his attack.
Ghirahim will walk towards you while blocking in two directions. This is the best attack to get, as you can immediately attack his open area and get in a few slashes.
Ghirahim will walk towards you while licking his sword. He will block your first two attacks, but then he is vulnerable. One method to deal with this attack is:
Ghirahim will summon a horizontal circle or vertical semicircle of darts around you. Do the appropriate spin attack to send all the darts flying back at him; this does a small amount of damage.
Ghirahim will summon a circle of darts around himself and fire them at you in three sets of five. There are two ways to deal with this attack. The first method is to slash each set of five as it appears to cancel the attack; this does not leave Ghirahim vulnerable and is slightly slow. The second way is to get close to Ghirahim and keep doing horizontal slashes. This will sometimes cause him to do the following attack, the Leaping Strike. If you hit him after that attack it will despawn Ghirahim's darts.
It looks you can hit him when he summons Dart Circle or Triple Dart but it actually doesn't deal any damages.
Ghirahim will leap into the air, and after a second or two land with both swords down where you are standing. After he lands, he is vulnerable while he pulls his swords out of the ground. You can dodge this attack by just backflipping once while he is in midair, then slashing him as he recovers.
Getting behind Ghirahim does nothing and stunlocking is misinformation. Look at the next "Stun Length" part.
His stun length is not always same. It's affected by his HP and whether he used Imminent Attack or not. The Imminent Attack always sets the stun length short, 5 hits maximum. However, when he has a lot of HP left and did other attacks (Feint Attack, Sword Lick or Leaping Strike), you can hit more than usual. After he took significant damages (about 2/3 HP left), it never happens. So it only happens when he didn't use the Imminent Attack in 1st or 2nd cycles.
This method is slower than playing this section normally.
Save at the bird statue when you reach the first outdoor area. Activate BiT and BiTWarp using the Lower Academy statue. Link will appear in a partially loaded room; to reload, exit through the door on the left.
You'll need to defeat the Dark Lizalfos; however, since you haven't triggered the Dark Lizalfos cutscene, he is stuck in a wait state. If you try to hit him then he will keep teleporting back to his initial state, making him invincible. To kill him, stun him with the Gust Bellows or the Whip, then use a spin attack and final blow. Then continue through the door that opened.