Dragon's Bane Trophy Glitch

Well here is my update.  I had pretty much done the completionist method, playing through most of the side quests and clearing nearly all of the areas pretty early on.  So I wasn't particularly far in the story by the time I was level 20+.  So after trying to get the trophy to pop by fighting the last Dragon and failing to get the trophy, I played through the main story for awhile then went back, killed the dragon and the trophy popped.  Don't know why it made a difference but maybe some coding was interferring or some such.  Anyways, it seems to have resolved itself but that would have really sucked if I had not saved and manuvered thusly and had to play a new game just to get 1 trophy.