Glory was a god, not a demon. Buffy couldn't take her without the hammer, either. Since some of the demons that Buffy fights are stronger than she is, and Buffy is stronger than Spike, and Spike fights the same variety of demons Buffy does, he's pretty much got to be fighting demons stronger than he is once in a while. (Plus he's killed two Slayers one on one, and presumably they were stronger than he was, just as Buffy is.) Presumably he wins fights against stronger opponents the same way Buffy does - by being a better/sneakier/dirtier/more inventive fighter than they are. While superior strength obviously gives someone an advantage in a fight, it's not the only factor affecting who wins.