I'd vote for the slide hammer too. I used a old-fashioned one where you have to drill holes in the metal to fasten it to the panel. After the crease was pulled out, the holes were covered in fiberglass body filler.
Now, they have nice ones that spot weld pins to the metal to affix the slide hammer. When you are done, you shear the pins off. Eastwood carries them (around $400
like this ). You may also be able to rent a set up from a tool rental shop.
Good luck.
