Cyberdimension Neptunia: 4 Goddesses Online (EP)

I got this trophy shortly after starting the Guild Rank 5 quests while doing the "Magma Diver" quest on Hestie Crater. Once there, you'll notice near the top of the map there's a small path jutting off to the left (the furthest one down). There should be an event there (if not, don't worry and just come back later). Once you walk towards it, you should get an event showing the four candidates finding a hot spring. This will pop once they enter the hot spring.