Formidable: (1) Arousing fear, dread, or alarm (2) Difficult to undertake, surmount, or defeat
By both these definitions, Glory is quite the formidable opponent. She's the only one of the baddies that had Buffy and her people fleeing in terror. And they were so desperate for a way to hurt her they had to go crawling back to the Watcher's Council. And Glory herself was almost impossible to kill. It was only because Giles was willing to kill Ben, and stopping Glory's plan still resulted in Buffy's death. I'd say that makes her a far more dreadful baddie and much more difficult to kill than the Turok-han, which beat her good at one point, but she was still eventually able to kill just using her fighting skills.