The Mezzanine

I think you are alone in that assumption. It's a troll hammer. Why would the Slayer have any special powers with regard to a troll hammer?

It's canon that Angel is stronger than Spike, and that Spike's victory in 'Destiny' was against the odds. It's also canon that Buffy is stronger than Angel; he says so to her face, plainly and unambiguously, in 'Sanctuary'.

Buffy is, therefore, significantly stronger than Spike.

Although the strength difference was undoubtedly exaggerated in 'Blood Ties' for the sake of a cheap joke at Spike's expense.