Galadriel is of course a powerful witch, and all witches are bad. She tries to stop the fellowship (it was her that sent the dragon to the mountains, but he was defeated by Gandalf), but then orcs come (randomly) from the Misty Mountains and assail Lorien and the fellowship. Boromir is killed. The orcs are driven off. Galadriel humbles herself and helps them.
Something to echo the Battle of Five Armies, which men of Dale will be more familiar with.
And... just sayin'. Yeah. No need to change the scenario if this is a stupid idea.