Actually I'm not sure I could do a straightforward shrapmetal spam and win in the arena with a solo character even at level 721. I tend to find at present after all the dragons are dead that my characters are down about 1/3rd of their HP at present...However the hop into the stands technique (jump into the stands, go right to the back row. That has a vertical wall in front of it that protects you from most of the dragons allowing you to take them out 1-2 at a time as they fly over it) would probably work at your level. I was able to take out the 20 gold dragons using that technique at level 90 or so.
--Chandor