Whiff has a negative connotation, particularly in the context of baseball, as suggested in the sentence. It means that Jennie will be most likely outmatched in the game.
EXPLANATION
Whiff in the context of baseball refers to when a batter swings and misses at a pitch and strikes out. The word whiff comes from the sound the bat makes when swinging through the air and it not making contact with anything. So, given that context, it would really imply that Jennie would be defeated against those "best hitters" in her game.
