Sam is playing by the Doctor's rules and not killing people. Even extras. Between the taser he's got and a nice right hook, he doesn't really have to kill anyone. They get some good licks with a pipe or a well placed fist but Sam is used to fighting demons and worse. The extras don't pose much of a challenge on their own but the sheer numbers occasionally do. None the less they end up unconscious and he ends up alright by comparison.