At the beginning of each phase, sprint forward and do a running jumpslash to attack Scervo, then do a vertical spin attack. One of two things will be happening:
This can happen one of two ways: either the jumpslash and the spin attack landed, or Scervo blocked the jumpslash, started a slow attack, and the spin attack stunned him. Either way, stab until Scervo is knocked backward on one foot, then close distance with a jumpslash. Repeat this combo until he is knocked to the edge.
This happens if Scervo blocked the jumpslash and started a quick attack. If the spin attack does not stun him, he's about to attack, so shield bash his attack to stun him, then stab as above.
Following this strategy will defeat all three phases of Scervo. There are small differences between phases; in his second phase, Scervo will sometimes have a slight pause before a direct stab, making the shield bash timing harder. In the third phase, Scervo almost always blocks the first attack. But the same strategy works for all phases.
After obtaining the bow, go to the main deck and shoot the targets on the lifeboat and the mast, but DO NOT strike the timeshift stone. Go back inside and save at the statue, then activate BiT. Perform a BiTWarp with the Bazaar statue and you'll end up in a partially loaded Boss Key room. Shoot the timeshift stone through the ceiling vent, then deathwarp using bombs. You'll respawn at the statue you saved at, but the Sandship will be in the past time state, allowing you to continue into the dungeon.
With good movement, it is possible to make an earlier cycle in the piston room belowdecks. This saves about 10 seconds.
The key to this strategy is to run past the ladder before whipping the first post, as that sets your angle back in the direction you need to go when you land.
The tentacles are most easily eliminated using fully-charged arrow shots; Skyward Strikes should only be used if you run out of arrows. Tentacles will always drop arrows if there are not already two arrow drops on the stage. It takes four felled tentacles to raise Tentalus from the water.
The hitbox on Tentalus' eye is rather strange. Try to set your aim on the top part of Tentalus' eye while it's closed, then shoot as she opens her eye to hit immediately. If you miss, aim again while sidestepping left or right to avoid her attack. It is possible to hit her while she is swinging, but easier to hit her once the strike has landed and her head stops moving.
Once stunned, it takes five strikes to cause Tentalus to recoil. Spin attacks and Skyward Strikes do not do extra damage. Rather than swinging as fast as possible, it is slightly faster to time your five slashes.
Between rounds, you can stand in a corner next to the wall, and no tentacles will be able to spawn underneath you. Then you can pick off the next four tentacles and repeat.
Once you reach the higher deck, shoot Tentalus' eye immediately, then hit her eye five times again until she recoils. Wait a couple seconds and hop into the water. You'll respawn at the other end of the deck, and Tentalus' eye will open as her hydra hair starts to attack, giving you a small opening to shoot her eye again. This shot must be done quickly; it is recommended to use the A button to shoot rather than the quick charge with the C button, as the quick charge takes too long.
If you miss the shot, you can backflip into the water and try again, or just fend off the hydra hair with your sword until you have an opening.
It is possible to reach the area where you fight Scervo from the main deck. Climb the main mast and take the zipline up to the second mast. Drop down to the area of the deck with Nayru's emblem on it; there will be a chain fence there. You can then use a bomb to Highflip over the chain fence and into the partially loaded bow area.
Once you reach this area, you are stuck until you reset or die, since the door isn't loaded.
It is possible with a precise bow shot to hit the switch on the mast without going back to the main deck. In speedruns, you have to return to the deck anyway to hit the lifeboat target, so this isn't used in runs.
In the boss key room there's a Beamos blocking a small passage where you're supposed to shoot a arrow. By performing a precise shot it is possible to shoot through it without first destroying the Beamos.
As you need to to activate the timeshift stone anyways, this is arguably slower than just killing the Beamos.