決め球 means best pitch. 決める is to decide sth, (here it' about the outcome of the game) 球 is obviously a ball, but here it means pitch in baseball. therefore, 決め球 is a best pitch you can do, that could determines the outcome of the game.

きめ球 is a baseball term. Also it means "the shot that wins the game". So the literal translation would be: It's impossible to use that to win (the game).