When the boys have Cameron shooting snow basketballs, they have one ready. Yet they constantly have one ready, he can throw these snowballs back-to-back, despite that they would require a significant amount of time to form, at their size.
When the teen boys counter Mandy with basketball star Cameron, his snowballs could be easily dodged. Using his basketball skills requires a overhead arching shot, which works fine with an unmoving basket, but the kids would have plenty of time to move to not be hit. Also, the large basketball-sized snowballs would take a significant amount of time to build, during which time Mandy could have countered.