Emerald Weapon
Just equip at least two members with the Mime command & have the other member equipped with the Knights of the Round summon as well as the W-Summon materia. Cast Knights of the Round twice & have the other two members of your party Mime the attack.
When the rotation comes back around to the person without mime command, DON'T ATTACK! Wait for one of the members with the Mime command to come up again & have them use Mime again. You can keep alternating back & fourth between the two until Emerald Weapon is dead.
Also be be sure to have one of your members equipped with the Final Attack + Phoenix Summon (or Revive) materia combination. Emerald Weapon has 1,000,000 hit points, I've counted & when it begins to run low it uses it special attack which takes of 9,999 from your entire party.