Here's how it happened:
* Goku and Broly were engaged in a fierce battle.
* Goku used his "Spirit Bomb", a powerful energy attack that absorbs energy from its surroundings.
* Goku fired the Spirit Bomb at Broly, and it struck him directly.
* The impact of the Spirit Bomb was so powerful that it completely obliterated Broly.
It's important to note that "Punch" is not a character in Dragon Ball and doesn't exist within the franchise. So, it's not possible for Punch to have killed Broly.