I've straightened 4 rims now with the exct same problem you have. I never had success pounding them back with a mallet, wood block, etc. This is what I do: I have a large bench vise mounted to a heavy bench that does not move. I loosen the spokes in the affected area (4-6 of them). You'll notice that some are loose and some tight. I heat the rim with propane to the affected area only and not too hot. I place the rim's damaged edge in the vise just to the side of the center of the damage. I tighten the vise very tight and then pull from the top of the rim towards me. Then I move the rim a little in the vise at a time and continue to pull from the top (about a half inch or so on each side of the apex of the damge. This method is fairly easy and there's not much chance of cracking the alloy. Once done I lightly file/sandpaper the are to smooth and sharp edges. Usually you can't tell there was a ding.