I've been doing Paintless Dent Repair for a living for twenty years now and I've heard every snake oil method of getting dents out. The only way a dent that size is coming out is by applying pressure to the back of the dent.
Take the rear panel off and put your hand in there. You'll be able to feel the dent. Push with the palm of your hand and see if you can get it to move. Sometimes you'll get lucky and it will pop back out. Other times it only comes out 75%.
Find a good PDR tech in your area and they should be able to finish the job. While they are in there, they can push out the smaller dents. Call your local dealership and ask who they use to do PDR on their used cars, every dealer has a guy. Good luck!