This weird glitch only works in the Japanese version of the game. When you're fighting any boss, choose to escape 8 times. Each time will fail, but if you do this procedure, for the remainder of the battle your physical attacks will always be critical hits! It doesn't matter what you do in between the failed escape attempts; you ARE allowed to do other things (such as cast spells) in between each escape attempt, but be sure you can survive getting attacked freely for each escape attempt.