Here are my thoughts on the battle outcomes and reasoning, I'd be happy to hear yours!
I rate each girl according to the relevant stats in a belly punching battle - abs, punching, endurance/toughness (basically pain tolerance).
The ratings go from 1 (worst) to 5 (best).
Battle 1:
Emma Watson: Abs - 3.5, Punching - 2.5, Endurance - 3
Avril Lavigne: Abs - 2, Punching 3, Endurance - 2
Outcome: Avril can't take much in the belly, Emma Watson wins after 4-5 punches as Avril can't continue.
Battle 2:
Ariana Grande: Abs - 4, Punching - 2, Endurance - 3.5
AnnaSophia Robb: Abs - 3, Punching 3, Endurance - 3.5
Outcome: Close call, but Ariana Grande's abs endure and win her the match after 9-10 punches.
Battle 3:
Hailee Steinfeld: Abs - 4, Punching - 3.5, Endurance - 4
Hayley Williams: Abs - 3, Punching 2.5, Endurance - 3
Outcome: Hailee Steinfeld has stronger punches and stronger abs, Williams fights well but can't go on after 7-8 punches.
Battle 4:
Lily Collins: Abs - 4.5, Punching - 2.5, Endurance - 2
Kristen Stewart: Abs - 2.5, Punching - 3, Endurance - 3
Outcome: Kristen is tougher but her abs are no match, She can't take the pain and Lily wins after 5-6 punches.
Battle 5:
Selena Gomez: Abs - 2.5, Punching - 3, Endurance - 3.5
Victoria Justice: Abs - 3, Punching - 2, Endurance - 2
Outcome: Selena Gomez is tougher and her punches are stronger, Victoria's soft tummy is in too much pain and she can't continue after 7-8 punches.
Battle 6:
Miley Cyrus: Abs - 4.5, Punching - 4, Endurance - 4
Emma Roberts: Abs - 3, Punching - 2.5, Endurance - 2.5
Outcome: Miley Cyrus outclasses Roberts in all aspects, her stronger punches and abs win it for her as Emma cries her surrender after 5-6 punches.
Battle 7:
Nina Dobrev: Abs - 3, Punching - 4, Endurance - 4
Bella Thorne: Abs - 4.5, Punching - 3.5, Endurance - 3
Outcome: Close one, but Bella finally wins a hard one after 10-12 punches.
Battle 8:
Vanessa Hudgens: Abs - 5, Punching - 4, Endurance - 4
Ariel Winter: Abs - 3, Punching - 4.5, Endurance - 4
Outcome: Ariel gives Vanessa a run for her money with her punching strength, but Vanessa's rock hard abs endure. Vanessa makes Ariel give up after 8-10 punches.