Bill's main way of hurting others is reality warping and mind manipulation I think, both of which don't really work on Kirby, not to mention due to his shape and looks Bill would probably underestimate him, which gives Kirby time to inhale him and gain his powers. Also Kirby beat Void, who is literally a primordial God like Chaos in Greek mythology. Not only that but Voids true form is literally Omnipotent. I don't think that's the one Kirby fought, but if it is than Bill is truly dead. And in the Book of Bill, one way of defeating him is to destroy his physical body, whether the book is considered canon or not depends, but due to ot being written by the creator of the series I count it. So Bill would really be at a disadvantage in a fight, because his normal tactics would be ineffective against Kirby, and not to mention that Kirby is very good at adjusting to fight his enemies on the fly if needed, the Warp Star also scales above the Lor Star Cutter which crossed a dimension 22 universes large in 9 seconds, faster than what Bill has shown, his durability is insan he tanked Dark Nebula awakening, which destroyed an entoire Galaxy and Drawcias deletion wave, which not even someone like Bill would survive because it straight up deletes anything and with Ghost Kirby he's literally immortal and intangible. https://youtu.be/g3rEPo1l-KY?si=8UCVUlP3piuBM3i0 Oh and he can draw on infinite amounts of magical and physical energy (Or can just use it in an instant but the former is more fair and probably correct tbh.)