Emerald : go in with less materia, nothing can hurt u and u can even do it in under 20 minutes (without underwater materia)
i am sure all of u remember his AIRE TAM STORM ?
well , if u reverse the word "AIRE TAM" storm
u get MATERIA storm

the more materia u wear, the more dmg... with 2-3 materia each character, AIRE TAM will only do 2k dmg +/-
Ruby : go in solo (alone), then he wont "whirlpool" away your cloud and once the battle starts a few rounds, ressurect your team mates... he wont whirlpool again (wahah 3 on 1 now b****)
Ozma and Tiamat really based on your level
but once u hit really high level, these guys are easy , just long
No wonder, His bloody storm can sapu my teammates in a while. And it;s because a lot of materia lol.