I agree with Ken, heat is a last resort.
Don't beat it with a sledge, gently massage it back out slowly with smaller hammers. you have high spots around the low areas, they need to be removed simultaneously with the low spots.
With the amount of damage there, along with the rust you mentioned you may be further ahead with a new 1/4 or at least very experienced help.
Your rear body panel is also bent. I can see 2 areas of direct damage and 2 of buckles related to the direct damage. Also looks like maybe the deck lid was hit so maybe there are some gap/alignment issues.
It is so hard to tell from pictures.
Has something been welded over the damage before??
If so, it has made it much worse.