ewoks vs stormtroopers

First, the surprise did last for a moment as the Empire, overconfident as they were, dispersed into the forest to directly attack the Ewoks. That is why they are running - they are getting out of a big clearing and moving to attack the enemy and not just running away.

Second, A rock thrown from 20 feet up is going to hurt no matter what armor the Empire had. We never actually see stormtroopers go do to any Ewok weapon that did not have the potential for a ton of kinetic force such as the bolas although getting beat by 5 furry Ewoks with sticks can't be fun.

Third, the Empire was winning the battle until Chewie captured the AT-ST and used it against the Empire. It was only then the battle actually turned. You can't say just because some Imps were down before then that they were losing. Clearly the Ewoks started out well but were losing before the AT-ST help.

Lastly, this whole Ewok/Wookiee thing I think is sometimes overblown. All that was really changed was 3 1/2 feet taken off, which does not make the Ewoks less vicious. They may in fact be much stronger than their height would indicate. No one would have been upset with a wookiee battle, but take 3 feet off and suddenly it is ridiculous?