Umm, emerald is the one underwarter right ?
It's got this annoyng attack, the upper art goes up and does massiv damage.
But wait ! This is funny, it' does umeber_of_materia_on_the_char x 1111 points of damage :]
Oh, and be sure to get the underwater materia, no time limit when fighting emerald.
Ruby on the other hand is a BIT difficult. Try to get All Lucky 7 ( HP 7777 ). I'v menaged go set my max HP as 7777 , then the char. attacks nonstop for 7777 a time, when finished just heal up and use W-Summon with Knghts of the Round.
Or go to
http://gamesfaq.com ad check other tactics :]