Ancient gods have been done before, but have they ever been defeated this easily?
For the most powerful ancien greek god, Zeus was a bit of a pushover and seemed less powerful than your average demon.
It's previously been established that god-strength is dependent upon each god's number of worshippers. That's why Lucifer was able to kill a bunch of pagan gods so easily back in season five. Those gods weren't worshipped much anymore, so their power levels were much diminished. Besides, even the most powerful of beings on this show seem to be killable with the right weapons.