But to answer the original question, without seeing the damage I would take this approach.
[img]
http://www.imagestation.com/picture/sraid225/pc358365c47a5b1e8d9ce6a72452beb94/ea387981.jpg[/img]
Place the dolly on the low areas of the dent and push up on the dolly. Then with the hammer lightly tap the surrounding high ridge that surrounds the dent.
This is called hammer OFF dolly and you dont hit the hammer directly on the dolly.
