Ruby Weapon / Emerald Weapon / Sephiroth

Location: Corel Desert

To fight Ruby Weapon firstly you must have defeated Ultimate Weapon (the weapon that flies around the world map).  After defeating it and Diamond Weapon's death, you should see a red thing sticking out of the sand at Corel Desert.  Make contact with it with either the Highwind or a Gold Chocobo and the battle will commence.

Fighting Ruby Weapon

You may need:
1x Knights of Round materia (Lv 1+)
1x HP Absorb materia linked with KOR materia
1x Mime materia
1x Ribbon
1x Mystile
1x W-Summon materia (Optional)
1x Hades materia (Optional)
1x Phoenix materia linked with a Final Attack materia (Optional)
1x Magic Plus materia (Optional)
1x Shield materia (Optional)
1x Enemy Skill materia with Big Guard (Optional)
1x Restore materia with Regen (Optional)

Begin the battle with one member alive (preferably Cloud) and the other two members KO'd (that way Ruby cannot suck up the other members with Whirlsand).  Ruby will summon his tentacles behind you.  Quickly cast Big Guard or Hades if you wish, but summoning Knights of Round instead will be safer (as the tentacles and drain your HP/MP).  After summoning KOR on the tentacles Ruby will take back his tentacles, and then may cast Ultima as a counter attack.  If you've equipped Mystile armor it should most likely miss, but if it does hit the HP Absorb will heal your character back to health.  Now your character should not perform any other attacks except for miming the KOR you've casted at the beginning of the battle.  Miming KOR for about 8-10 times will defeat Ruby Weapon, and you'll receive the Desert Rose.  Of course there are other ways to beat him, such as miming limits or equip the Counter/Magic Counter/Counter Attack materia strategy, but I've shown you the safest way.

Ruby Weapon attacks
Big Swing
Whirlsand
Shadow Flare
Comet2
Ultima
Ruby Ray

Ruby's Tentacles
Absorb HP (causes paralyze)
Absorb MP (causes Toad)

Location: Ocean Abyss (Underwater)

Maybe the toughest enemy in FF7, this Weapon lurks underwater, near the Corel Desert shore.  To fight it firstly you must have access to a submarine.  After acquiring the submarine from the third Huge Materia Quest, dive the submarine to near the bottom and you'll come across a green thing swimming around.  Make contact with it and thr battle will commence.

Fighting Emerald Weapon

You may need:
1x Knights of Round materia (Lv 2+)
1x HP Absorb/MP Absorb materia
1-9x HP Plus materia
1-6x MP Plus materia
1x Underwater materia
1x Phoenix materia linked with Final Attack materia (both Lv 2+)
1-3x Mime materia
1-3x Sprint Shoes
10-99x Megalixirs, X-Potions, or Turbo Ethers
1-3x Mystile (Recommended)
1-2x Zeidrich (Recommended)
1x W-Summon materia (Optional)
1x W-Magic materia (Optional)
1-3x Enemy Skill with Big Guard, Angel Whisper (Optional)
1x  Restore materia with Regen (Optional)
1x Shield Materia (Optional)
1x Bahamut Zero materia (Optional)
1-3x Revive materia (Lv2+, Optional)
3x Counter Attack, Counter, Magic Counter materia (Optional)

The best characters to use to fight Emmy probably is Cloud, Barret, Cid because of their powerful limits.  Remember not to equip too much materia on any one person, as Emerald Weapon's Aire Tam Storm attack's power depends on how much materia you've equipped (eg. 6 materia equipped = 6666 of damage).  And also the Underwater materia will rid the 20:00 limit.  Begin the battle by casting Big Guard or Regen.  Summon KOR materia and skip to the party members who has Mime materia and keep miming the KOR.  If your characters are killed by Emmy, you'll have to start the process again.  If your KOR runs out of summons, start miming your limit breaks.  You fight the whole battle by miming your limit breaks without summoning KOR if you want, but the battle could go longer.  Another method is the classic Counter/Magic Counter/Counter Attack method which you link them with materia such as KOR, Phoenix, Ultima.  Emerald's eyes, which can rejuvenate, drains your party's HP/MP, but they have low HP.  Make sure the member with Phoenix + Final Attack has at least 180 MP if he/she gets KO'd.  Emerald Beam attack damages 3/4 of your members current HP, Emerald Shoot damages one member around 8000 and despells, Revenge Swamp damages all members around 3000 and Aire Tam Storm damages depends on how many materia each member has equipped.

Emerald Weapon attacks
Emerald Beam
Emerald Shoot
Revenge Swamp
Aire Tam Storm

Eyes
Blue eye absorbs MP
Orange eye absorbs HP

Location: Inside the Planet

Sephiroth is a reasonably easy boss to defeat, even though its the final boss.  To reach him, reach the center of the northern cave and after defeating Jenova*Synthesis you'll meet Bizzaro*Sephiroth (all 8 members can fight Bizzaro), and then fight Safer-Sephiroth with Cloud's team.  The interesting thing is that the higher level Cloud is, the more HP Sephiroth will have.

Fighting Bizzaro*Sephiroth

You will need:
Your normal equipment of mixtures of Comet, Ultima, HP Plus, Counter Attack, Enemy Skill, Throw, Bahamut Zero, Hades, Mystile will be enough but having Knights of Round will end the battle quickly.  Having Ribbons and Mystiles will be essential when fighting the last form of Seph.

When you fight Bizzaro*Sephiroth, it'll have 5 parts: Main body, Master Head, Left Arm, Right Arm, and Core.  This battle is easier then it seems.  Cast Wall or Big Guard, or Regen, then start blasting him with everything you've got: Coin Toss, Ultima, Comet2, Pandora's Box, Hades, Bahamut Zero, you name it.  Every time you kill 2-3 of his body parts you'll be asked if you would like to switch party members.  I recommend using your best-equipped party during the entire battle because the battle will end even when only 3 of your members are KO'd.  One Knights of Round summon will instantly dispatch him.  Sephiroth can restore his HP with his Bizzaro Energy, and has an attack that damages and cause status ailements which is called Stigma.

Fighting Safer-Sephiroth

After defeating Bizzaro, Cloud's party will fight Sephiroth's final form in the heavens.  Your summon times will be reset, but not your HP/MP.  Begin the battle with the normal Wall/Big Guard/Regen/Dragon Force stuff.  Now throw EVERYTHING at him.  I recommend tossing 60,000 gil at him and summoning KOR, which will instantly dispatch him.  This battle will be quite easy and won't be a challenge, but his Supernova attack damages 9/10 of your members current HP, and his Heartless Angel attack turns all members HP to 1.