I cant find any videos online of dogs killing wolves for the most part (though I dont doubt this happens), but most of these videos even like sviatogor shared look like some kind of set up. I can find videos of two wolves killing and literally grabbing a pitbull and running off with him. Flock guardians are never meant to be alone, and they are just meant to be a big enough of a problem for the wolves that wolves are deterred from messing with them as wolves dont always want to risk injury. Wolves are extremely fast and have a powerful bite, they also have more stamina than dogs. a wolf the same size as the dog, shouldnt have much problem making work of him. 

whats interesting is that modern humans also show signs of domestication, probably our cro-magnon ancestors would also make quick work of us.