Melphyx (Retinite in the SNES and PlayStation versions) is a boss that appears in the Sunken Desert in 600 AD.
Strategy
When fighting Melphyx in the Sunken Desert as part of the quest to restore Fiona's Forest, Marle can cast Ice II to harden the sand-body of the creature, leaving it vulnerable to physical attacks. While this is useful, Magus' Ice II tech and Frog's Water II tech are equally as useful. However, bringing along all three of these Water/Ice Elemental characters ensures that no matter whose turn it is to act, the party can effectively damage the creature. Because, once the body is hardened by Water or Ice, it is extremely vulnerable to physical attacks.
When struck with a physical attack, its defense increases. In order to maximize damage, the player must strike it with Water or Ice before every physical attack. Ayla, Crono, and Robo are able to do the most physical damage per strike, so having at least one of them is a must.