Mission 13 is located on an elevated area at Area 3 on the left side of the area. Go left when first approaching area, passing the brick walls. There is a ramp to access and get up it. Mission 13 should be behind Agent 3 when above the ramp.
Another way of reaching Mission 13, head to the 2 ramps on the right side of the area, make it to the platform and there should be a small wall in front of Agent 3. Going passed the small wall, go around the center of the tower, and behind it shows a kettle to Mission 13.
Shifting splatforms are used through this mission and often found near grating or other obstacles. These splatforms are used as a type of puzzle for Agent 3 to solve. While most of the splatforms are horizontal in nature; three of the four platforms that lead to the Zapfish are vertical. The first and third splatform move at the same rate, while the second moves at a different rate.
Beginning Area
Walk off the Spawn Point and down the ramp. There will be a grate. Turn into a squid to pass through the fence. There will be a big moving Splatform. Walk onto it. Paint the platform and swim to the other side of it and jump off it onto another platform. Destroy the crates and splat the two Octotroopers. Then walk up the ramp, and press to Super Jump at the Launchpad.
Checkpoint 1
Jump off the platform with the checkpoint. There will be several Splatforms to pass through. Cross them, like at the beginning. Splat the Octotroopers and Octocopters before they can splat you. On the last one, turn right, paint the small wall and junp off the Splatform onto it and climb up it.
Checkpoint 2
In this Checkpoint, Agent 3 will come across some Octocopters which are the only enemies on this checkpoint. Splat them, and head upwards on moving platforms. The last Moving platform trvels under a grate, form into a squid to pass the grate and the Launchpad is on the last platform, which isn't in motion. Towards the end of the walkway. There you will see three boxes, one of which contains the Sunken Scroll.
Final Checkpoint
Go through the first Splatform and kill the Octocopter and Octotrooper. Destroy the crates, including the one with armor. Go through another Splatform before heading to the three shifting horizontal Splatforms. Ink the Splatforms and then swim up and get the Zapfish.
Heralded by loud explosions, the 'Great Octoweapons' quickly stormed the Inklings' central stronghold. Victory for the Octarians seemed all but certain, but due to a plug being carelessly pulled from its socket, their hopes were dashed.
At the second checkpoint, beside the large wall. When landing on the checkpoint, you will be facing a large wall with shifting splatforms and two Octocopters overhead. Rather than engaging them, go on the path to the right. Around the corner of the wall will be three large crates. Destroy them and collect the scroll.