I've never tried that. I prefer a challenge. Anyway:
Emerald: Equip all characters with Mime materia and Final Attack - Revive materia in a double-slot. To get three Mime MASTER it, then MASTER the new one. Give one character Knights of the Round. When the battle begins, use KOTR and Mime away. Final Attack - Revive combination is for when your characters get killed (which they probably will). 12 Knights of the Round kill this guy.
Ruby: Give Mime and Final Attack - Revive to all. Give one character Hades and one Knights of the Round. Paralyze Ruby with Hades so it doesn't counter-attack with Ultima when you use Knights of the Round. Use Knights of the Round and Mime away. Paralyze Ruby again when he un-paralyzes and repeat. 10 Knights of the Round kill this guy.
Safer Sephiroth: Give one character Destruct materia, one Knights of the Round, one Restore - All and one Mime. Use Debarrier to destroy Sephiroth's Wall (he'll use it first things) then Knights of the Round. If the first one doesn't kill him (very likely) then pound him with physical attacks, Comet2, Ultima, limit breaks or Mime KOTR. If you don't destroy his Wall he will take two KOTR and some extra moves or three KOTR to kill. He isn't too hard once you figure out strategies.
Hope I helped.
I'm finished here...
Special thanks to Yossy666 for the avatar and signature images.