The Apex Killer Whale fight is unlocked once you complete all the side quests in the Caviar Key region. When it's unlocked you can find the Orca in the Sea World-esque pool in the middle of the region.
Depending on your evolutions, this is possibly the toughest Apex fight. The Orca is level 40 and has really high health, and there aren't many other animals to eat to regain your health.
In the fight, it's key to stay out of the orca's jaws. Use RB to dodge as the orca lunges at you, and attack from the bottom and sides when possible. Use the Y button to activate evolution when the orca is open to attack. Use LB to tail whip and stun the orca, then bite 2-3 times immediately after before evading. Below is my fight with the Apex Orca.