Magical Combinations
Comet 2 + MP Turbo + Quadra Magic + W-Magic
This way you will stroke the enemy with 160.000 damage! Enough to take out even the last boss in the CD 3. To make it work perfectly you should have 255 of magic and even some Magic Plus materias.
Demi 3 + Quadra Magic + W-Magic and MP Absorb + HP Absorb
You will inflict 80.000 of damage using only 80 MP! And you drain out of the enemy 800 MP!
Reflect + Demi 3 + W-Magic
If you don't have too many Demi materias mastered you can put all your characters in reflect than attack yourself with Demi 3. The attack will be reflected 3 times to the enemy!
Physical Attacks Combinations
There comes a point in the game where magical attacks become useless. In these situations you must use normal attacks. To improve them you can equip some magic materias with added effects too.
For example, when you equip added cut with mug you attack the enemy plus you steal them and you do this two times!
Added Effect
Want to kill an enemy in a single blow even if he doesn't get enough damage? Equip added effect materia with destruction materia. When you attack him the sword will inflict death that kills the enemy instantly like if you used the death magical spell
Other Combinations
Magic Counter with Heal. This way when the enemy attacks you, you automatically heal yourself.
Magic Counter with Death. The enemy that touches you, dies...
Counter with Death Blow + HP Absorb. When you are attacked you counter with a powerful blow that if hits it will get 10% of the HP damaged from the enemy!
Finally
Counter with Flash. When an enemy attacks you the counter attack will kill him automatically again!
As you can see there are endless powerful combinations and I don't know them all! If you know more please comment so I can learn from you. Good gaming!