Walkthrough[edit]
You will start on a platform with an Octopod in front of you moving around in circles. When you move forwards it will notice you. It will run towards you, and upon reaching Agent 3, explode, doing a lot of damage. Shoot at it before it reaches Agent 3 to prevent this. On the sides are some crates. The orange crate contains a power egg. Shoot ink at the wall ahead, and swim up it in squid form to reach the checkpoint.
You will enter an area with many Octopods, which stand on crates. Shoot the Balloon Fish, it will explode and splat all of Octopods and destroy the crates. The crate at the back left hides another crate containing some power eggs. Shoot ink at the walls and swim up, similar to earlier. You will encounter more Octopods and crates on the way to the large platform at the top. There is a large wall in the way now. Shoot the Balloon Fish, similar to earlier to cover it in ink, and swim up the wall. Enter the launchpad by following the 
prompt on screen.
In the next area you will encounter a new enemy, the Octoball. They roll around, bouncing off walls and destroying crates. To defeat them you must get them caught in your ink, and then you can attack them. Keep following the you will encounter a group of three Octoballs, which will roll down the ramp towards you. Defeat them by shooting ink in front of you for them to get stuck in and then attack them. Keep moving forwards and you will reach the next checkpoint.
There is a wall with a Squee-G. They will wipe ink, making it disappear. It does not take damage. To reach the grate above you must cover enough of the wall so that it cannot completely destroy your path before you make use of it. Swim up quickly before the path disappears. The Squee-G cannot hurt you, so it is OK to attempt again if you fail. Walk along the grate. Although swimming on grates is generally poor advice, if you move you camera and look down there is a platform below. To collect the Sunken Scroll fall through the grate by entering squid form and break the crate, and use the launchpad to jump up onto the grate again. After the grate there are three crates, with an Octoball hiding behind them. Break the crates and defeat the Octoball the same way as earlier. Cover the walls in ink and swim up them to reach another launchpad leading to the next checkpoint.
In the next area you will encounter more Octoballs and Squee-Gs. Defeat the Octoballs as they notice you. There is armor on top of one of the Squee-Gs. Collect it so that you can survive taking more damage. This area can get a bit chaotic due to the obstacles, so it is best to try to only get the attention of one Octoball at a time. Don't worry about the Squee-Gs, they cannot hurt you, but they can make it more difficult to trap the Octoballs. Go up the tall platform closest to the wall and shoot the Balloon Fish, which will cover the walls in ink, except for the bottom part which you will still need to cover. Swim up the walls to reach another checkpoint and launchpad.
Enter the launchpad and you will land on top of a pillar of crates. The Octoballs will notice you and destroy the crates. Try to splat the Octoballs before you reach the ground, which will be covered in enemy ink from the Octoballs. Enter the launchpad and you will reach another checkpoint.
Swim up the wall to a thin path. Some Octoballs will suddenly appear. Shoot at them and they will fall off the path since it is so narrow. Jump of the end of the path to the platform below. An extremely tall wall will be in front of you. It is too tall for you to ink now. You will be able to ink it later. Destroy the crates on the right of the wall, enter the launchpad leading to the final checkpoint!
In front of you is an Ink Cannon. Hold to use it. You can aim like normal, and shoot exploding projectiles with 
. Cover the large wall ahead in ink thoroughly, and also shoot the Twintacle Octotroopers. Move to the wall quickly and swim up, watching the moving part of the wall. You may need to cover some of the wall with you ink again since the Squee-G will remove some of the ink. Shoot the Zapfish's shield and walk into it to finish the mission.