EMERALD WEAPON: You can find Emerald Weapon swimming around underwater, usually stopping to rest over the underwater crater, or near the sunken Shinra sub.He has four independetly functioning laser beams that can absorb your HP and MP points, and a number of special attacks capable of doing 9999 points of damage to all partymembers. You have to do a million points of damage as soon as possible. Knights of the Round can do about 126,000 points of damage in one turn. All you will need is Knights of the Round at level 3 or higher two mimic materia(make the first one to reproduce) and W summon materia. Emerald Weapon will make 9,999 points of damage to you. You will need to return from the dead by using Final Attack+Phoenix.
RUBY WEAPON: You will have to use the same basic strategy here, but Ruby's initial attacks tend to suck away two random party members, leaving one character to face the challenge. You have two options here. One is to give character Knights of the round, W-summon, Final Attack+Phoenix or Revive, Ribbon and the Mystile. You have to start attacking Ruby when her stalks appear. Your first priority should be stalks. Ruby will kill you a few times but your Final Attack materia will keep you alive. The second option is to fight Ruby until she allows two or more characters to stay in the fight.