The Sea Monster Attacks!

The Sea Monster Attacks! is the twentieth mission in Mario Tennis Aces, taking place in Savage Sea Bay.

Layout
Mario and Toad enter Savage Sea Bay, where they feel a large presence at the boat. Gooper Blooper then appears, forcing Mario to fight him.

In this mission, Mario must defeat Gooper Blooper within 300 seconds. Gooper Blooper starts by slamming down one of his forward tentacles as he fires balls of his own ink, which Mario must knock back; unlike previous bosses, missing a ball will simply produce an ink blotch that obscures the screen rather than subtracting five seconds from the timer. Gooper Blooper will occasionally alternate between which forward tentacle he slams down, and fire a few Zone Shots on some occasions. Once Gooper Blooper is out of health, Mario must fire a Zone Shot at his exposed mouth. Gooper Blooper will then simultaneously slam down either both of his forward tentacles or both of his back tentacles, alternating between both sets of tentacles several times. He will then perform a low sweep with one of his forward tentacles followed by the other, which Mario must jump over both times. He will then repeat his ball-serving attacks, except he will fire more Trick Shots and more often fire ink balls on the side of his tentacle opposite the one Mario is on, forcing him to jump and perform some Zone Speed to reach the ball. After Mario fires another Zone Shot at Gooper Blooper, Gooper Blooper will again slam down his two sets of tentacles, except this time finishing with an attack where all four tentacles are slammed down, though Mario can retreat to an open area to evade this attack as Gooper Blooper crosses his forward tentacles. Additionally, at the end of his tentacle sweeping attack, Gooper Blooper will sweep both tentacles simultaneously and clap them together; Mario must jump over either tentacle to dodge the attack. Gooper Blooper will then repeat his ball-serving attacks once more, though very little changes from the second phase are present.

Afterwards, Mario attempts to obtain the fourth Power Stone, only for it to be stolen by a corrupted Wario and Waluigi.

In-game text

 * "Defeat the boss of Savage Sea within the time limit!"