To unlock the Superior Shot achievement, you must earn 3 balls from one shot in Peggle 2.
To earn a free ball in Peggle 2 this you must either score 50,000, 75,000 or 125,000 from one shot, get the ball in the free ball bucket at the bottom of the screen or win a coin flip after missing a shot completely.
The easiest way to get the achievement is to shoot your ball into a clump of pegs after you have a 2 or 3 times multiplier, hit a purple peg and get the ball in the free ball bucket.
Alternatively, you can just score 125,000 from one shot and get 3 free balls.