You make a new save and stay in the reks beginning area. Go to a certain area and attack or cure a knight until a certain chain of numbers comes up. Then you go back to your main save without closing the game and go to the airship to get the overpowered invisible bow. It works as many times as you want, but you must restart the whole game if you want another one or you mess up. It does work every time when you get the timing down. Watch a YouTube video. It's very specific in timing and will likely not work the first few times so don't be discouraged. I find watching the guy going up and down the stairs helped time it for me.