||Fire (Halves), Water (Immune)
||Water Gem (x2)
The Achelous can be found in the underwater sections of the Mount Gagazet Cave. Being in water, it is weak to Thunder-based attacks, although you are restricted to Rikku, Wakka and Tidus, so magic might be an issue. After being targeted, it will begin to charge and after charging three times, it will perform a Sonic Wave on the entire party. Other than that, it has a basic attack.