If the hero moves away from the altar, and you destroy it...and the hero doesn't get knocked out the whole battle, can you still get 100%? (even though the chances are 1 in 10000)
Other than the Grand Warden, you do not have to defeat enemy heroes.
You obtain one star for destroying 50% of enemy buildings.
You obtain a second star for destroying the enemy town hall.
You can obtain a third star by destroying all enemy buildings.
Walls and Traps do not count as buildings. Hero Alters do count as buildings. And of course the Grand Warden is included as previously mentioned.
In practical terms, you're not often going to be able to destroy all the buildings without first taking down the Heroes, but they are not required.