Watcher's vs Celestials
This is ofcourse saying the Watcher's break their vow.
I realize most of you are saying ONE watcher against ONE celestial would be no fight, but I saw a comic once where they showed the Watcher homeworld and there looked like there were hundreds, maybe even thousands of them, and there are only about what, 10-12 Celestials? So, I think the sheer number's would help offset the Watcher's inexperience and weaker power's.