It is one of the strongest enemies in the Lifespring Grotto due to its boss-like HP. Even a Thundaga spell might not be enough to defeat it in one attack. Killer Sharks sometimes appear with their lesser White Shark brethren.
Killer Shark can be strong, but Lifespring Grotto should not be entered until near if not the end of the game. Because of this, the player should have most magic spells. Thunder spells can be used, but it is unlikely to kill them in one hit since Killer Sharks have over 1000 HP. One easy way is to Temper and Haste the Warrior (who should likely be a Knight by now) using a Black Mage (who should likely be a Wizard at this stage). A hasted and tempered Knight can easily do over 1000 damage with each hit, and Haste and Temper are both stackable.