It's so frustrating because all it would take for Oda to make us believe that Kid's attacks are actually doing damage would be a few panels here and there explaining things.
-It's completely reasonable to think that Kid can infuse his giant metal fists and contraptions with armament haki, but Oda doesn't tell us shit.
-In fact we don't even KNOW if Kid even has armament haki because Oda doesn't tell us shit.
-Kid's laser could be special and use it's electromagnetic properties to cause internal damage, but Oda doesn't tell us shit.
-We could've gotten 1 page, just 1 page during the last chapter of act 2 where we see Kid talking with Killer that he's interested in learning Ryou like Luffy did.
No one questions that Law's attacks do damage because he does the bare minimum to explain why they work, but with Kid all we readers see is a bunch of metal being thrown at one of the most durable characters in the series, and a laser that we have no idea if it's even any different than Queen's or Franky's.