Plasmarine

Plasmarine was a jellyfish-like boss that haunted the inner bowels of Jabu-Jabu's Belly in Oracle of Ages.

Link had to fight this beast to acquire Rolling Sea, the seventh of eight Essences of Time necessary to restore power to the Maku Tree and destroy the sorceress Veran. Plasmarine had two attack modes: blue, in which it hunted down targets and shot bolts of electricity at it, and red, when it attempted to electrify itself to hurt nearby intruders. Link destroyed the creature by using the Long Hook to flip Plasmarine onto its own electrical bolts. As the creature died, it left the Rolling Sea unguarded, which Link immediately added to his almost-complete collection of Essences.

Strategy
Plasmarine is arguably the easiest boss in Oracle of Ages, compensation for a challenging dungeon. By waiting until Plasmarine attacks, Link would then use the Long Hook to swap positions with Plasmarine, forcing it to take its own attack. Its red attacks would injure its blue form, and vice versa.