Well, he does fight demons, some of which are stronger than Slayers, so I don't think it's totally impossible that Spike could take on a group of Slayers and survive. It's also possible he'd end up dust in the wind. Honestly, provided that both parties are competent fighters, this is something that's totally dependent on the needs of the plot. If you need the protagonist to beat a group, you come up with a way to make it plausible. If you need them to get killed/captured, you come up with a way to make that plausible, too. Angel may have beat Spike out in raw strength, but Spike nonetheless captured him in In The Dark.