Starting in the entry of Area 5, head to the right and use the entryway. It's blocked off by a grate, in which Agent 3 can simply clip through it in Squid form. There is a Propeller-Lift. Use it and get up the wall it stops at. Now get up the grate slope. There is an Inkrail, but don't take it. There is a wall behind the Inkrail. Using the wall, there are two dry sponges. Use the sponges, and get up the small wall.
Starting Spongy Observatory, there is a Balloon Fish, which can wet the four sponges, getting Agent 3 to another Checkpoint.
There are two Octostamps to fight off. They're standing in sponges, so if they do attack, they'll shrink the sponge. Splat both of the Octostamps to reveal a launchpad.
An Octotrooper guards a platform which to get across, Agent 3 has to go through two sponges. If the Octotrooper starts shooting, the sponge will shrink. Be careful of staying up on the sponges to get across. Throw a Splat Bomb to the Octotrooper and it'll stop bothering Agent 3 from getting across.
After wetting the two sponges to get to the next platform, Agent 3's is going to have to get across a sponge alley, guarded by five Octotroopers. There is an Inkzooka to help get across easier. Not using the Inkzooka can be challenging, due to enemy ink Shrinking a wet sponge. If chosen to not use the Inkzooka, take out the Octotroopers to make it easier, but be cautious they can shoot it at a high fire rate, making a wet sponge shrink faster.
Another Balloon Fish can be used. There are also three crates on the right, one crate holding armor. Still going, another Balloon Fish can be found, and also an Octobomber. Take out the Octobomber, so that it won't shrink the sponges.
Yet another Balloon Fish can be found wetting four sponges, it can also be used to take out the two Octobombers. Wet two more sponges to go to the next checkpoint.
Using another Balloon Fish, Agent 3 has to make their way up on the mission but will meet about two Octobombers. One is just flying around but will chuck a Splat Bomb if found, and one guarding an area, which can shrink a sponge if caught, toss a Splat Bomb at the sponge. Once Agent 3 passes them, the mission continues.
Wet the sponge and get on it. Start putting ink on the wall to swim down and reach the launchpad. It is also possible without putting ink on the wall, just perform a swimming squid jump off the sponge, and face toward the wall while still in Squid Form, Agent 3 will automatically activate the launchpad, going into the final checkpoint.
A small crate holds an Inkzooka, going into the area will trigger an Octoling fight. Using the Inkzooka, Agent 3 can insta-splat the two Octolings. Once both of the Octolings are defeated, a launchpad will appear. Take it to get the Zapfish.