In the Dive! missions, you'll have to go into battle to defeat three opponents in two specific locations: Hydro 16 or Holly Hedges.
The first destination is indicated on the in-game map. For the second location, you will have to go to the hangars below the large dam. They are located to the south of the map, between Slurpy Swamp and Misty Meadows.
Map
Choose the notable place Hydro 16 will allow you to meet fewer people and succeed slowly but surely. For aggressive players, the fastest will be Holly Hedges.