All strategies mentioned beforehand work great, but it is possible to beat them without all that. I was able to beat Emerald and Ruby without having any of the special materia such as KOTR or Mime or anything along those lines. I just went insane using Cloud, Yuffie, and Cid and just kept hacking away using their best mulit-attack limit breaks. I was lucky and was able to defeat him with 3 seconds left on the timer. I didn't have Underwater materia either.