You don't take 28 of damage with your opponent in 1 of health everyday.
FUUUUCK!
There are a lot of decks now that are built to do exactly that. You never know what turn the 20+ damage is coming but if he pulls it off, there's nothing much you can do. So you have to scare them into using pieces of their combo as removal. If a Shaman uses Rockbiter early, you know they are going to have a problem later.