Spinal Phase

Spinal Phase is the seventh stage in Octo Expansion's finale.

Gameplay
Agent 8 must defeat a partially sanitized Agent 3 in a boss battle.

Agent 3 is armed with their Hero Shot, s, and has a large arsenal of Special weapons at their disposal. They will always enter the arena by a Super Jump and activate upon landing, with the exception of the  phase. Additionally, Agent 3 can dodge roll at any time as if they were landing with Drop Roller. Agent 3 will use the Hero Shot and Splat Bombs in all phases of the fight, but uses a single type of special exclusively in each phase. It should also be noted that like the player, Agent 3 is able to heal themselves overtime, accelerated if they swim through their own ink.

Phase 1
Agent 3 will land into the stage with a. After that, Agent 3 will fight normally. Agent 3 does have access to during this phase whenever their special is charged.

Phase 2
After taking sufficient damage, Agent 3 will Super Jump back to the spawn point on the UFO flying over the arena. Agent 3 will Super Jump into the stage and immediately activate a, trying to corner the player before setting it off. The Baller can be stunned with sustained fire, but cannot be destroyed until the duration automatically detonates it. The rest of the battle continues normally, though Agent 3 can activate another once their special is once again charged.

Phase 3
Super Jumping back onto the UFO, Agent 3 will remain on top of the UFO, firing off the in between volleys of Hero Shot fire and Splat Bombs. The UFO is difficult to reach with the Octo Shot's fire, though can be reached while standing on one of the platforms, but it is low enough to throw a Splat Bomb on top of it.

Phase 4
Agent 3 in the final phase will start by using the up to three times in a row. There is a period of rest between each Splashdown where Agent 3 is vulnerable, if enough damage is dealt to Agent 3, they will cancel out of their attack and resume normal combat. Agent 3 can perform their successive Splashdowns again once their special is charged.