When I tell you how to kill some of these tough bosses I'm expecting that you know to be in a good level (60 is good) and have the Knights Of the Round materia. So if you do not have it, do not challenge some of these bosses. Also you may want to buy HP plus materia from Cosmo Canyon and fill up any empty materia slots with them.
Can you say easy? Even before I got Knights of the Round I killed this guy before he did anything serious. Just cast some good spells at him, doesn't take much.
This guy has quite a few body parts but he's still not all that tough. Just throw out the best you have and keep your people alive and well. Make sure somebody has a restore materia (With Cure3) paired with an all materia. Also give somebody Knights of the Round and somebody Mime and this should be an easy fight.
This final boss is no walk in the park. However give somebody the Debarrier spell and preferbly a Wall spell. Make sure somebody has Mime and Knights of the Round. Cast Debarrier on Sephiroth then try to get wall up (Not really essential). Have Knights of the Round paired with MP turbo or W-summon. Cast knights of the Round then ahve sombody mime it. He should die. But in the event that he doesn't, pair Ultima Magic with Quadra Magic and cast that every round while somebody heals and somebody else mimes it. It shouldn't be TOO hard to beat him, jsut watch out for Super Nova >:)
One tough mother. Don't even mess with him without Knights of the Round. But if you want an easy fight, fgith until you master Mime and get a second copy. And if you ahve a LOT of free time, fight until you get master the second Mime mater so you have THREE mimes (Or jsut master Knights of the round). The best startegy is to ahve the three mimes. Have everybody get a mime and one person get a KNIGTHS OF THE ROUND. Now cast Knights of the Round and have everybody mime. IT'S THAT SIMPLE. Bt if you only have two mimes, master Knights of the Round so you can cast it five time and let the other two mime it. He shouldn't live through that.
He maker Safer Sephiroth look like a cute bunny rabbit. He will suck down two of your player so pump one of them up. You can try the Knights of the Round and Continuos MIME approach. Pair Knights of the Round with W-summon (Or turbo Magic) then equip a mime. Pair Final Attack with a Revive materia (So when you die you will automaticly revive yourself :) ) and pray to god. Throw what you've got at him and hopefully you can beat him. The only other thing I can tell you is, May the Force Be with You.