You will want to move the hammer around the while hitting the high spot. You need to work out both the low spot and the high spot together. As probably mentioned before the low spot is causing the high spot. Your dolly needs to be held very tight with outward pressure against the low spot while you hit the high spot with your hammer.
You need to relieve the stress on that metal from the high/low spots before you do anything. Heat shrinking the high spot at this point will yield poor results, as the stress is still in the panel holding the low spot in.
IMO the shrinking hammers and dollies that are out there are junk, they destroy the metal. They fatigue the metal, weaken it making for repairs that later down might occur impossible. Picture meat tenderizers and what that piece of beef looks like after. The modules that make up that steel will just be pounded to death, stay away from the gimmick tools.
When you get to the point that you do need to shrink that metal, which you do, the BEST method is with a shrinking disc. Its the most controlable and fastest method and does NO damage to sheetmetal.
But for fixing that one dent, its not really needed, you can get by using other methods that work on the disc's basic princple.
If you have a DA sander that can be switched into grinder mode you can use that to shrink the metal. If you dont have one that can go into different modes by moving the weight on bottom of the sander then this method WILL NOT WORK.
Take a dull piece of 120-220 sandpaper and run it over the metal. How this works is simple, it hits the high spots in the metal, heat builds up JUST in those high spots caused by fricition. Run your sander over the metal for around 5 secs, then cool the metal with a wet rag, if you DO NOT see steam you didnt heat the metal enough. Continue using the sander until you see steam when you cool.
You will probably find you need to do a few runs of heating and cooling, just the DA sander and the wet rag.
These two links show some metal work using nothing but simple hand tools and the shrinking disc, the end result is a repair with no filler what so ever, you could simply prime it and paint it. Hopefully you might get some ideas on what to do...Eric
This one explains what is going on and shows the tools used.