Ok here it is my tried and tested method of beating Emerald weapon easy.
First make sure you have at least two mimes and one nights of the round, and of course underwater materia.
Make sure one of your charachters has only 2 materias, and if possible, connect KOTR with hp absorb.
Now when you go into battle imediatly summon Kotor and get the others to mime, if you have less than three mimes, just skip those that don't have it. After a while if the person with only 2 materias is at full health(9999) when he uses Air Tam Storm, that person will go into all Luckey Sevens and Emerald will be dead in a few minutes.
It worked for me
Also if you have cait sith with his Slots Limit then use him in your party(make sure he has a limit)and use Slot, repeatedly push square(note not in the PC version)and get the combo that is instant death for the enemy(I think it is Triple Skulls).
I don't know if this works but I will try it on Ruby soon.