Your goal is to get all the EXP and AP as fast as you can right? No problem. Go to the most difficult place in the entire game which is the crashed genilka. Now equip Cloud and his buddies with Counter Attack materias. At least 4 for each. This way each one will strike the enemy four times after being attacked.
You can equip all the character with the 4x cut materia. If you have the Luck Plus materia you got at the Battle Square in the Gold Saucer use it to increase the critical hit ratio of the blows.
Forget about magical attacks. The enemies here are just too strong for them. You would need four or five attacks to make slight damage and that would take just so long...
Don't even think about summons! They are too slow to make it profitable...
For the fastest battle possible equip all the characters with the Flash attack you get when you master the slash all materia. You only need one blow to kill the enemy!
If you want to get the sources to increase your abilities up to 255 you should equip Yuffie with the Conformer and the Morph materia.
Just keep in mind the enemies are the toughest in the entire game!
Don't go for Dragon Force, Big Guard or any other spell. They waste precious time. If you can't take out the enemies without needing to heal and increase the status in battle you should get more HP Plus materias.
The characters you should use all the time are Cloud + Cid + Barret. Their weapons and limits are the best ones.
