Got to agree with Milice - this is not a "dent" - it's a bent derailleur hanger, with threads and the same ALUMINUM as the frame. So if you pound on it you will likely ruin the threads or produce a crack. Bending it back will not be an easy task, as aluminum is not ductile like steel. If doable I would not advise trying it on your own.

Last resorts for risky repairs are not advisable for non-mechanics anywhere but in the sticks. It's best to have a mechanic experienced in frame repair, or better yet a frame builder, do the work.

As for the derailleur, of course you can buy one that does not come with a hanger. The hanger goes with the frame nowadays, not the derailleur.