Chances are, you will get this during your first round of play through the various Masters. At the time of this writing, I am personally four short of the goal after having played 3 of the 5 masters. The Optional Objectives appear a the begining of each level and can be viewed in the pause menu.
Each level will be something different, but a vast majority of them are for achieveing a level's Ace Score, clearing all pegs off the board, or beating the level with 3 (or more) balls still available.
There's more than enough optional objectives to get 30, but for the other achievements the best advice I can conjur is to experiment. If you can't complete a level with Bjorn for instance, why not try Gnorman (or Zoidberg)? Or try more indirect shots off of the wall.
Lastly, and especially if you are aiming to have X number of balls left or are trying to clear all pegs, place your shots well nd time it to angle into the bucket for easy free balls.