Octoling Assault is located on the second level of the tower in Area 3. When on the second level, find platforms resembling a staircase, then the grate path will take you to the mission. It takes place in a modified version of the multiplayer stage Blackbelly Skatepark.
Leaving the start area, Cap'n Cuttlefish sees some Octolings, which will attack. These can easily be splatted by Seekers. From the start to the second checkpoint there is a line of Power Eggs. Ink the ramp and climb up it to reach the second checkpoint. On the ledge of the central tower, there is a box that contains an Octoling. Now there is a line of Power Eggs up the ramp to the top of the central tower. In the orange box on the top of the tower, there is a piece of armor. There is also another Octoling on the ground. On another ledge in the wall, there are some Power Eggs. Drop down the side of the tower and climb the ramp to the third checkpoint.
After you pass that, on the ramp to the left, there is a canned special, which contains an Inkzooka. Octolings are shooting at you during this. On the grates to the right, there is a box that contains some Power Eggs. There is also a line of Power Eggs running to the next part. On one of the smaller ramps, there is a box that contains another piece of armor. There is also an Octoling trying to splat you when you approach. There is a path of Power Eggs to the Gusher. However, before you activate the Gusher, go on the path to the right. At the end is a box which contains this mission's Sunken Scroll. Then, run back, activate the Gusher, and burst the Zapfish out of its shield.