Gusher Gauntlet
From Inkipedia, the Splatoon wiki
Gusher Gauntlet is the fourth mission in singleplayer Octo Valley. It is the first mission of the second area after the first boss, The Mighty Octostomp!. The mission is named as such because of its heavy use of Gushers.
Octo Valley
The kettle for this mission is in the far right corner of the zone, on top of a platform.