When fighting Amams one will want characters with high Magic power with Magic-boosting accessories and Lightning damage-reducing accessories (such as Spark Rings), so be sure to have Hope and Vanille in the party. One will also want the Guerrilla (SAB/SYN/RAV) and Relentless Assault (COM/RAV/RAV) or Tri-disaster (RAV/RAV/RAV) Paradigms.
Cast Deshell and Slow on the Amam while the SYN casts Haste on the party. When this is complete, switch to either Relentless Assault or Tri-Disaster. Just keep attacking the Amam, and recast the debuffs if needed.