I'd pull the taillight of course, then work on it from the inside. I have a crap-ton of drifts and punches of all sizes and shapes, both metal and wood. I'd start at the edges of the biggest part of the dent with something having a rounded nose and start tapping it out. I find a 2# drilling hammer works well for enough muscle.
When I got that one close I'd get some help and have someone hold a belly curved dolly against the outside and finish reshaping it. You might need to heat this, that's a pretty sharp dent.
Once the ugly one is more-or-less back in place, I'd reverse the process and have a helper hold a dolly on the inside while I went after the pimple-looking things with a flat nose brass drift from the outside, again trying to flatten them back into place. I'm nobody's idea of a body man so if you get better advice take it. But I think I could work this back to where bondo would make it look respectable.
