Killer is a lot stronger than people give him credit for. His main fight in wano was hawkins, however he was stomping hawkins until hawkins started using hostages. And even then, Killer basically 1 shot him when he took back Kid's voodoo doll. He didn't even need to use his strongest attack, sonic scythe, in order to win.
Sonic scythe was capable of damaging Kaido. Given Killer's speed and endurance(which is pretty underrated, given that he took some powerful attacks from Linlin), imo he should at the very least be on par with Jack or Cracker.