you just need to use the right strategy, then it becomes really easy. its best if you go into the battle with 2 characters dead. have Final Attack linked to Phoenix, the W-summon materia, and the Mime materia linked with Counter. in battle when his hands are in the ground use hades and KOTR with W summon. hades will cause Ruby to be inflicted with Stop. then use mime over and over again till he's dead
Omega Weapon on the other hand, is slightly more complex than that