Say for example there is a level 2 Team Valor gym nearby with a 2500 CP Snorlax and a 500 CP Vaporeon.
One day I get together with a bunch of my Team Mystic friends and we all decide to battle this gym at the same time. Every single one of us beats the Vaporeon, but we all lose to the Snorlax.
If enough people battle a gym simultaneously, is it possible to lower a gym's prestige to zero without beating the gym leader?