Something that might help is to make a sandbag. Find a leather purse, fill it with sand and hang it from the window sill above the dented area so that the sandbag rests tight against the area you are working - on the outside. Now when you smack the panel on the inside the metal will meet with resistance instead of the entire area flexing. You might have to rig an additional brace outside to keep it tight against the fender.