Take the small ramp on the left and furthest from the entrance of Area 5. There is another ramp, but don't use it. Perform a Swimming Squid Jump to reach the other side where there is an Inkrail, but don't take it either. Go on the grate path and use the Gusher. There's the Kettle for Octoling Onslaught. It takes place in a modified version of Saltspray Rig, a multiplayer stage.
The player will spawn on a checkpoint on the right side of the map. Nearby is a big wooden crate with nothing in it. Once the player walks out of the spawn point and after Agent 2's dialogue, a Kelp Octoling will jump onto the crate and start trying to splat the player. Like always, you can defeat this Octoling or make a run to the checkpoint, which is to the left path of the box.
The player will be near the bottom part of the rig with a rectangular-shaped area. Once the player jumps off the edge of the path, a normal Octoling will jump to a high part in the middle of this area. The player is able to grab an Inkzooka Special in one of the small crates on the left. Once you are done here, the next checkpoint is on the Bottom Area, where the lifts are actually absent.
Once the player reaches this area and moves forward, another Octoling will spawn. On top of one of the oil tins here is a crate which contains a suit of Armor. Obtain this, and move on to the section near the top part. Here, the player can defeat the Octoling and take a nearby Inkrail to the unreachable side.
Here is where the Sunken Scroll can be obtained. However, once the player reaches here, another Kelp Octoling will appear as well as a normal Octoling once the player reaches the crate with the Sunken Scroll. Grab the Sunken Scroll, defeat the Octolings and go back to the middle part to the next checkpoint.
Another Octoling will spawn and on one of the small platforms, and another crate containing Armor is located nearby. Grab this Armor and the final checkpoint is after walking up the ramp to the final area.
One more final Octoling will spawn, and this is where the Inkzooka can be used if the player hasn't defeated some of the other Octolings yet, as they will Super Jump here. Once the player defeats one, one last Kelp Octoling will appear. Either defeat these Octolings or splat the metal containers, swim up them, splat the Gusher, then jump off onto a platform with the Zapfish. An Octoling will be waiting for you there. Splat it, then burst the Zapfish's shield and take the Zapfish.