Big Mom is still significant enough to have been a threat to Roger in his era from what we're told. I imagine she'll be able to beat Rayleigh via soul manipulation whilst Shiki distracts Roger until the two can double team the Pirate King.
Imo Roger is a good bit stronger than all the Yonkou (I think WB got weaker in his old age) and Rayleigh was as strong as a yonkou, so PK and Rayleigh probably take this with mid difficulty. Something like an Admiral vs Luffy.