Let's share some Materia Combos that make the game very easy!
I'll just start with some improvised stuff:

THE KNIGHT-REVIVER:
W-Summon = MP Turbo
Phoenix = Final Attack
Knights of Round = Final Attack
Magic Plus

THE DEATHBRINGER
Ultima = Sneak Attack
Ultima = Steal As Well
Ultima = Quadra Magic
Ultima = MP Turbo
W-Magic

And just for fun...

THE SEPHIROTH
Revive = Restore
Earth = All
Ice = All
Lightning = All
Fire