I agree. Do NOT use a porta-power on that dent. You're likely to stretch the metal and make a new "anti-dent" that stands proud of the original contour.
Get the truck on a jack stand and remove the wheel to give you some room to work. Starting point would be to use a low crown dolly and smack the dent from the inside with it near where the object that caused it hit. Doing that will flatten the crease a bit. Then work with the hammer from outside near the high spots with the dolly held tightly under the low spot near where you strike with the hammer. What you want to do is bounce the dolly off the back side and have it rebound to hit the low spot. That's called off-dolly work. Like Hotrod Lincoln said, avoid striking the dolly directly. It's pretty amazing how the metal will work it's way back into shape. Take it slow and work your way around the damage.
It probably won't be paintless, but it'll be pretty close. You might be able to live with polishing up the marred spots and not have to repaint.
