How about a weapon that when it strikes a living creature, all the connections between its brain and its muscles are reversed. That way, when the brain tells a muscle to contract, for example, it expands instead. The effect would be fairly quick but would take over the body gradually starting from the extremities inward. That way, the creature flails about uncontrollably until the effect reaches their heart when then starts pumping backwards. Also their lungs would work backwards and they'd breathe out when they needed to breathe in. That would suck all the oxygen carrying blood back out of where its supposed to go. The creature would then suffocate and die even though it wasn't being smothered by anything.