How do I create a smooth slope?

I made a 3D Scan of a house and I am trying to remodel it as a low poly version. At the moment I am struggling to get a smooth slop where the grass is. Is somebody able to help me? Link to the blender file:

Thanks a lot!

Use Proportional edit, select vertex (vertices, if needed) or edges, GZ keys to move Z direction and by scroll mouse up/down adjust influence radius (circle on screen which tends to either go behind the screen surface or shrink so small that one can not find it :wink: ).