My two theories:
1. Science and magic are opposites and in many opinion the greatest advancement science has that magic doesn't is the creation of electricity through lightning. And if you notice through out the OUaT universe, no one magical actually touches lightning (except for Jafar and Amara in the last two episodes of Wonderland but I don't really count that since it was an extremely powerful spell that changes the very laws of magic). Rumple summons lightning with the wand in 2x09 and directs it to the well, but never actually touches it. Jafar manipulates the weather in Wx08 but never touches the storm cloud. So Tamara's tazer could have been a regular tazer that killed The Dragon and August since lightning kill the magic within people. Greg and Tamara also used the same technology to torture Regina almost to death.
2. Since Greg and Tamara worked for Pan and he could make things materialize through the power of belief (for example: the fake Pandora's box). So maybe Pan created the tazer with the belief that it can kill magical creatures and then gave it to The Shadow so it could travel to Earth and leave in a place where Tamara would find it (maybe it was in a box labeled "From the Home Office."