Upon entering Area 4, Octosniper Ramparts can be found on the left side of the front corner of the area. To reach it, there is a Gusher near the Tower. Activate the Gusher and Octosniper Ramparts can be found.
Make your way to the launchpad when you spawn in. After you land you will have your first encounter with an Octosniper on a high ledge. With four Octotroopers stationed around it. Either, stay out of its sights by hiding behind a wall or in your ink, or go head-on towards them while dodging its attacks. Once you have splatted them all, a Launchpad will appear at the top of a ramp behind where the Octosniper was. Take it to progress ahead towards the next Checkpoint.
Once you land, you are met with two Twintacle Octotroopers and a single Octosniper. Swim past its sights in a path of your ink and take out the Twintacle Octotroopers. Now you have to submerge into your ink on a moving platform to slip past a net fence and splat the Octosniper.
Now onto Checkpoint 3, yet another Octosniper is guarding the area. Take it out, but slip through the grate to obtain the Sunken Scroll. Climb back out and continue over grated walkways while splatting Octotroopers. But be aware of two Octosnipers on a moving platform, guarding a Launchpad. Wait patiently for the right moment to attack, and splat the Octosnipers to take the Launchpad towards the next Checkpoint.
There is a base, guarded by Twintacle Octotroopers and an Octosniper in the Center. Take out the Twintacle Octotroopers, and ink the platform the Octosniper is on to get into range to splat it to reveal a Launchpad there. Take it to reach the mission's penultimate Checkpoint.
Now you are met with some platforms with sponges that are needed to cross them. But at the end, there is an Octosniper. Keep in mind that its attacks will shrink the sponges you inflated. Continue forward and splat the Octosniper while heading for the Launchpad.
The Zapfish is located at the top of a series of ramps that are guarded by four Octosnipers and a few Octotroopers. You can swim up some walls to get past them easier. Once you have reached the top, you are now able to collect the Zapfish to complete the mission.