Looks like a perfect case for the heat//cold technique. Look it up.
What happened here is that the metal has stretched under the force that was applied to make the dent. When you heat up the metal above a certain point, the microstructure can rearrange itself and get rid of the dislocations inside. Then by cooling it very quickly locally, the metal will shrink and might be able to completly remove the dent.
Basically, you heat it up with a heat gun around the area but mostly where its dented. Then, you take one of those compressed air gas cans to clean computers and you spray the dented area with the can upside down (essentially shooting liquid nitrogen). This WILL take more than one trial. If you have an infrared thermometer, you can monitor the temperature and increase it for every trial.
Hope this helps!