Tentalus

Tentalus is the fifth boss encountered by Link in Skyward Sword. When Link tries to access the control room on the Sandship, Tentalus appears and nearly destroys the whole ship with its massive tentacles. Due to Tentalus's large size, Link cannot significantly damage or even reach the majority of its body; only when the boss's eye is within reach of Link's sword can legitimate damage be applied. To obtain Nayru's Flame, Link must best the beast in battle atop the crushed ship's deck.

Strategy
After opening the door to the Sandship's power room with the boss key, the battle begins. Fi notes that there seems to be a large lifeform attacking the ship from the outside, and Link must make his way to the deck of the Sandship. During his ascent, Tentalus's tentacles burst through the side of the ship, halting his progress. Link uses a Skyward Strike perpendicular to the tentacles orientation to cut them away; the rocking of the ship dislodges barrels that also oppose Link's progress to the ship's deck. After climbing to the deck, Link avoids Tentalus's tentacles and continues to remove them with perpendicular Skyward Strikes or arrows. Once several tentacles have been removed, Tentalus surfaces from the waves and attempts to damage Link with its arms. Link then shoots its large eye with an arrow, stunning it. While it is stunned, Link attacks it directly with his sword; by cutting off Tentalus's tentacle "hair", Link may receive arrows and hearts. After enough damage has been dealt, Tentalus breaks part of the deck, causing a few barrels to fall towards Link, and a crate also falls from the higher level of the deck. Using the crate, Link climbs up to the upper level, where the battle continues.

Tentalus continues to attack in much of the same manner and is damaged in the same way. However, instead of attacking with its tentacles from below, it attacks with its toothed tentacle "hair", which is repelled by quick sword slashes on Link's part. Alternatively, Link may jump off the deck once the hair appears. Doing so yields Link no damage, and allows him enough time to aim a shot at Tentalus's eye before the "hair" returns. Once Link defeats Tentalus, the storm clears. Link is then rewarded with a Heart Container and Nayru's Flame, upgrading his sword with enhanced dowsing abilities.

Trivia

 * Before going to the outside Fi tells Link to prepare. Since the whole ship was destroyed it is almost impossible to prepare in anyway.
 * Tentalus is a possible reference to Tantalus, an ancient Greek Mythological figure who was doomed to live in a pool of water as a eternal punishment for his wrongdoings.