Buy a decent panel beating set, included should be a shrinking hammer along with other shrinking tools, The aluminium used on series is quite britle whether this is due to age or not i'm not sure, one of the most important things to do is annealing the metal which involves placing oil onto the face of the metal and heating it from the rear until the oil disappears whether this is the same on defenders or not I don't know.
You can also make your own shrinking tools from old files, heating them up and bending them to the desired shape. I'll upload some pics of our kit along with some of the tools that we've made later on.
You will also need to keep working the metal back and forth, don't try to just welt the hell outta it in one go, do it slowly and methodically working your way out around the dents. Panel beating is a form of art in my opinion it takes time and patience to obtain a decent result that requires no filler although sometime it's unavoidable.
The kits come with various sized and shaped tools that'll help with larger areas aswell as getting into the nooks and crannies, Although decent sets can be expensive they are worth there weight in gold and have many uses and will save you money in the long run. It is also definitely worth investing in a slide hammer kit as these are invaluable tools with unlimited possible applications.
If you do need to fill any remaining defects, I would advise lead burning, this involves heating the aluminium and cleaning it removing all oxides using a stainless steel wire brush, apply a small amount of flux or Borax to help clean the aluminium, then apply your lead and shape it with thin metal spatulars and after letting it cool you can sand or lightly file any minor defects out.
