Aeris' Ultimate Weapon

Princess Gaurd, Aeris' ultimate weapon is located in the Temple of the Ancients. In the room of the time guardian, turn the hands of the clock so that one hand is pointing to the platform that you are on and the other is pointing to door IV. You can now enter this door. The Princess Guard is at the end of the hallway.

Aeris' Level 4 Limit Break

Great Gospel, Aeris' ultimate limit break, can be a very powerful spell. To get it, you must find a cave near Fort Condor that can only be reached by buggy, Blue Chocobo, Highwind, or Tiny Bronco. Inside you will find a sleeping man who will tell you how many battles you have fought. Speak to him when the last two digits of this number are the same to recieve a mythril. Take the mythril to the weapon seller by Gongaga, who was not there earlier on in the game. When you give him the mythril, he will let you chose a prize from one of two boxes. Open the box on the second floor to find Great Gospel.