Note that you will only ever face 6 of the 7 below Saints, as Gongora will kill whoever he attacks last in the cinematic battle on disk 3, due to that saint resisting when told otherwise. While this does not change very much, it is advised to let either the Black or White saint be killed, as they are generally the more difficult to kill due to their weaknesses being that of something other than the standard elements.
This battle can be a bit of a puzzler since all of the Saints can be damaged in only one specific way. Their weaknesses are as follows:
Red ---> Fire
Blue ---> Water
Green ---> Wind
Yellow ---> Ground
Black ---> Status Ailments
White ---> Light
Gold ---> Physical Attacks
Things are pretty simple here if you know what to do. Start by casting Petrify on Black if he's there -- three turns later he won't be. You can also drop one of the other elemental saints in a single turn with an -us spell if you have one; if not, two turns of an -a spell should do it. Several rounds of physical attacking by Kaim and Seth will take down the Gold one. The only tricky one is the White one if you don't have Shinus. Hit him with Shine or Shinus though, and he'll fall. Oddly enough he is also susceptible to Toxin. Independently, all of the Saints are weak. However, Grade 6 Magic Chant is a brutal attack that will hit your whole party. They can only do this when there's six of them though. Once you kill one of them, the rest of the battle is cake.
Alternatively, Prismus can take them all out at once.