I have modified the example a bit.
I have changed it to impulse at angle instead.
And if you use:
angle(0,0,ball.physics.velocityx,ball.physics.velocityy) you will get the current angle of the ball.
Then i deduct 180 degrees to get the opposite direction.
I played a bit around with the impulse speed. you can set that to what you want.
In the example I have set it to be dependent on the speed with witch it hits the mine. (by using distance instead of angle)
But play around with it and see what works for you.