The Balance Beam Boss is worth 15G and is unlocked by scoring 16.80 points or over in the Balance Beam.

To meet the required mark you will need to choose the Hard difficulty. It is not possible to get a score, high enough on the other two difficulties. I would advise practicing the routine several times before making a serious attempt. You must get a high percentage of Excellents and Perfects, it is key that you dont miss any steps. The dismount comes very quickly, it is quite easy to miss the final button command so be ready. A respectable landing may get you those extra points you need.

Should you still have trouble beating the required points, try using slow-mo in Olympic mode to slow down certain aspects of the routine.