Mission 8 is located on the highest tower on the north-western corner of the area. It takes place in a modified version of the multiplayer stage Kelp Dome.
Hop straight down from the spawn point and the grate, then an Octoling will jump from the center. Taking it out will make it easier to reach the next checkpoint. Follow the line of Power Eggs and you have reached Checkpoint 1.
Follow the line of Power Eggs to the center and another Octoling will come. Again, splatting it will make reaching the checkpoint much easier. You will be confronted by another Octoling, so do the same as before. Follow the trail of Power Eggs round and you will reach Checkpoint 2.
Follow the Power Eggs round to find another Octoling and splat it. There is another path of Power Eggs - follow it and you have reached the final checkpoint.
There is a trail of Power Eggs. Follow them around and an Octoling will try to ambush you from behind a potted plant. Then, climb the Inkrail up to the part you cannot usually reach and you will find another Octoling. Splat it, then climb the Inkrail to the grate and you have reached the Zapfish.