Once you have a creature or economy advantage over the Rushmaster, anything that extends the game tends to work to your advantage. Yes, they can wait another turn before throwing the second boulder at you if you reveal Brace Yourself, but forcing them to wait before attacking you still tends to work in your favor. Besides, you can respond by taking other defensive measures, and it gives your investments time to accrue value, whereas the Rushmaster doesn't have any creatures.
Similarly, forcing the Rushmaster to spend actions and mana on Teleport drastically slows down the rate at which she can deal damage. You don't NEED a creature with Intercept in order to be able to beat the Rushmaster, but they can certainly slow her down.