You can bulk archive (or refile/change todo etc) from within the Agenda view.

http://orgmode.org/manual/Agenda-commands.html#Agenda-commands

If you call Org-Agenda from within the buffer you want to archive you can temporarily restrict it to only that buffer and view only todo entries and filter for only DONE

C-c a < t N r

Where N corresponds to the shortcut for your DONE state (with default states it would be 2)

Then you'd simply need to mark all the desired headlines and bulk archive