Gusher Gauntlet
From Inkipedia, the Splatoon wiki
Gusher Gauntlet is the first mission in the second area in Hero Mode after the first boss, Mighty Octostamp.
Octo Valley
The kettle for this mission is in the far right corner of the zone, on top of a platform.
Walkthrough
The Sunken Scroll is at the final checkpoint on another grate not far from the Zapfish.