Am loving my replay through this.
Won the Chocobo Race and got the buggy and then went back to Junon and set off the alarm near the elevator to the underwater reactor, and have been levelling up there. Upto level 36 with Cloud, Barret and Red XIII now and am about 2000AP off Bolt3, Ice3 and Fire3. About 1000AP off Regen too. Never been this highly levelled so early on in the game before, it's kinda
Stobzilla, best way to get Aeris' 4th limit would be to do this (This is how I got it a few years back):
1. Obviously you start off with Healing Wind. Use a hyper on her and make sure she is always in a 'Fury' state, so her limit bar fills quicker.
2. Once you get the Cover materia, equip that on her so she starts to take more damage than she normally would, by covering for other party members.
3. Simply keep doing this until you have used the limit 8 times, and you unlock Seal Evil.
4. Now let Aeris strike the finishing blow in every battle, and she'll have soon killed 80 enemies. Once you get the Enemy Skill materia, equip it to Aeris and learn Matra Magic, and have her cast it to kill off battles which have multiple enemies. You'll have Breath of Earth soon enough.
Now, once you get past the Cave across from the Chocobo Ranch, go and fight in the forests. You will either fight 3 birds or 5 small things, but it is easy to gain your limit breaks as they do quite a bit of damage. Just simply repeat the steps above with Aeris and you'll have her 6th limit break in no time (Then you have to get the Mythril to get the Great Gospel).
The whole process, if done right, will take about 90 minutes.
Best armour to use to level up materia doesn't really matter, as most of them early in the game don't offer double or triple AP.
Best advice I can give is wait until you are on Disc 3 and then fight the Magic Pots, you get a shitload of AP from them and if you spend a couple of hours down there you can master a shitload of materia. Took me about 3 hours if I remember correctly to master the 21 green materia 2 times over so I could get the 2 Master Materia from the Huge Materia's which you store at Cosmo Canyon.