If you destroy an opponent's battlefield can you attack their units directly or can they just let you attack and soak the damage since it's already destroyed?

There is no way you can attack an opponents unit directly unless there's a card effect that allows you to do so. You can still attack a burning zone if you wish to but unless you have some special ability like Wilhelms, there's really no point.

Animosity forces a target unit to defend. Just another card that might have some use attacking a burning zone.