Why not use the physics behavior or this behavior?

You can do it without the physics behaviors but you'll have to do some math.

If ball is in the air then move it down.

If the ball hits the ground then...

1. find the angle of the ground.

-- one way to do this is just have different sprites for different angles or you could even use the angle of the ground sprites.

-- another way is to use two detector sprites. One a little to the left of the ball and another a little to the right. Then in a loop move each detector down till they hit the ground. Then just calculate the angle of the left detector to the right.

2. Move and roll the ball down the slope. This is where the math comes in.

-- the simplest way is of course to just use the move at angle action.

-- the amount to rotate the ball can be calculated with distance_moved/ball_radius*180/pi. And make it rotate counter clockwise if the slope is to the left (or the angle is from 0 to -90 degrees.).

That will give roughly ball motion. It can be improved instead by giving velocity and angular_velocity to give the ball momentum and calculating the accelerations on a slope for the angular_velocity. So basically the better you want it to look the more physics equations you need to use, which is something those behaviors already do for you. But it never hurts to learn how things work.