Celestia didn't die. She was knocked unconscious by Nightmare Moon's blasts. She woke up, weakened, and brought out the original Elements of Harmony to banish her sister to the moon... something she cried over but had to do in order to save herself and all Equestria. Twiley realizes that this is all a flashback.
So they could take out Celestia, a pony who could manipulate a freaking giant ball of raw plasma, like she was nothing, but a bunch of ponies just punching and using an artillery piece modified to create instant parties defeats an entire invasion... geez, no wonder they made Twilight a princess.