The two hardest bosses in the game are the optional Emerald and Ruby Weapons. They have a million hit points each and can deal out enough damage to take you down easily. Beating them takes stragey, patience and very high-level characters. Safer Sephiroth is a cinch comapred to these guys.
I suggest you take on Emerald first. To defeat this moster, you need to have your characters at least to level 80 or so, and use your three strongest characters. I beat Emerald this way -- I used Cloud, Yuffie and Cid. Insert your own character neams whenever you see them. The characters were equipped thusly:
Ultima Weapon --
Escort Guard --