When you go to the caves were the three ledgendaries are at battle them (except Mesprit it runs away) first get its HP down to a minimum of 10hp or less, then throw any kind of ball and do the following.

1. When it says so and so threw the whatever ball press an hold L and R.
2. When the ball opens press A.
3. When the ball flashes press b and hold it until the ball hits the ground then release ONLY A then start pressing L, R, B, and A.