For all of these encounters you'll need to have beaten the game and need 25 shards of the corresponding element.
Grunda(mineral) is in a cave 1 east and 1 south from the Desert Outpost teleporter, once in the cave head east and go down the two wind jumps, once at the bottom go north and he's under his skeleton.
Nara(plant) is at a statue 1 west and 1 north from the Hidden Village teleporter
Luxa(electric) is in his Office which is 2 south from the Cadium teleporter(not the lab)
Ventra(wind) is in her shrine 2 east and 1 south from the Drake Isles teleporter
Arqua(water) is in the middle of the screen 2 south 1 west 1 south from the Palmaya teleporter
Fona(fire) is in his shrine 1 south from the Ignitia(not south or west) teleporter