I did this by drawing a freehand mask around the mound but keeping inside away from the edge, but following the contours. I then feathered the mask by a few pixels so it grew to the edges of the mound - slightly further actually. This was to avoid a hard edge, and also to avoid adding to the slight halo you already have.
I then just adjusted the brightness for the mound - removed the mask and saved the image. You could just as easily have used the mask to darken the mound if you wanted more of a silhouette effect. I keep pushing this feathered mask technique, but it's a really handy way of dealing with problem areas like your mound, and you can apply virtually any filter you like within the mask, sharpening etc. Obviously you could use the same technique for any other areas in the pic to get them how you want.
