You'll fight all four elemental fiends in one battle here, in the order of Milon Z.., Rubicant, Cagnazzo, and finally Valvalis. As you defeat each one, it will switch to the next in line. The first three are vulnerable to a single Element and absorb all others (except Milon Z.. who is vulnerable to both Fire and Holy). Pound these weaknesses hard to win the fights quickly. Valvalis is neither weak nor resistant to any Element, so use your most powerful attacks. You may want to go back to the save point before moving on, because the next boss fight is much tougher.