Magellan’s Memento Explorer Achievement takes place in South-East part of Frostgorge Sound, northeast of Arundon Waypoint in the grawl cave. You can skip grawels easily, but try to dodge red circle immediately or you will die. Two Veteran Grawls guard The Makeshift Gate, and on the other side of the gate two Veteran Oozes are waiting for you. At the end of the Tunnel there is a Giant Blue Ooze (part of [Group Event] Defeat the Giant Blue Ooze) in front of the Grand chest.
Wooded Kingdom Moons in Super Mario Odyssey