After opening the big boss door on the Sandship in the Lanayru Desert, the ship will come under attack by giant tentacles. These tentacles can also grab Link, so don’t get to close.
Don’t waste time in the room that the tentacles broke through the floor as there is nothing to be done here. Instead, simply turn around and head down the path to the side of the room where two more tentacles will break through the walls to block Link’s path. Clear the path utilizing the Skyward Strike and keep moving forward and be sure to avoid the rolling barrels along the way.
Once Link reaches the top of the ship and exits through the door, Tentalus shows its full form and dives underwater thus beginning the first of two phases.
Phase 1:
It will then shove its tentacles up through the floor creating pillars that will try to grab Link and throw him off the Sandship. There are however, multiple ways to clear this phase; Horizontal and Diagonal Skyward Strikes will still cut down the tentacles, as will fully charged arrow shots, and perfectly timed bombs (very hard to pull off btw). NOTE: Vertical Skyward Strikes will no longer work while tentacles cut by using fully charges arrows will drop arrow bundles, so do not worry about running out.
After cutting off four tentacles, Tentalus will surface on either the front or one of the sides of the ship allowing Link to shoot him in the eye with an arrow (does not have to be charged). This will cause the boss to fall over. Here Link has a few choices, he can either cut off any number of Tentalus’ dreadlock like tentacles, or he can simply run under them to strike at Tentalus’ eye once more. After striking said eye, Tentalus will dive back underwater. Repeat the previous steps two more times to end Phase 1.
Phase 2:
A cutscene will begin in which Tentalus will lash out and start to destroy the deck that Link is standing on. During this, a block will fall down from higher ground. Run over to the block while avoiding the rolling barrels and climb up to the roof where Tentalus will appear in front of Link. Shoot an arrow into its eye while avoiding its tentacle lashes. Run in and attack the eye yet again but be prepared because after this Tentalus will gain a new attack.
When Tentalus’ dreadlock like tentacles grow mouths and start to attack simply attack away with Link’s sword as these are very weak and will die in one shot. They do have the ability to regenerate quickly, so when the boss takes a break to roar at Link shoot him in the eye with another arrow (again, it does not have to be charged up). Once Tentalus falls over, run under those dreads and slash away one last time to put an end to the Abyssal Leviathan.
Skyward Sword - Abyssal Leviathan Tentalus Boss Guide ,