Best party for this battle is characters with high Magic power with Magic-boosting accessories and Lightning damage-reducing accessories (such as Spark Rings). Hope and Vanille thus are good party members to have. Good paradigms to have are Guerrilla (SAB/SYN/RAV) and Relentless Assault (COM/RAV/RAV) or Tri-disaster (RAV/RAV/RAV).
The party should cast Deshell and Slow on the Amam while the SYN casts Haste on the party. The party can then switch to either Relentless Assault or Tri-Disaster.