Of all the super people that make up Netflix’s Marvel mini-verse, Luke Cage is easily the least vulnerable of all of them. He’s super strong, like Jessica Jones and Danny Rand, but his defining physical trait is his impenetrable skin. You can’t shoot Luke Cage — the bullets just bounce right off him. But Luke is certainly not unkillable. So as we head into the second season of “Luke Cage,” it’s worth looking again at how a bad guy might be able to take Luke out.