You could easily make EMP weapon from any old microwave oven. All you need is to take it apart, get magnetron, transformer, wires and button, make a 2.4GHz horn antenna (endless antenna calculators in the net) from sheet of copper, bronse, brass, aluminium, steel, mount magnetron to horn and assemble transformer, button and other staff in a gun-like manner. It is not a "killing every electronic device around" thing, but it perfectly works against drones and most cameras in plastic case, effectively damage WiFi input amplifiers and other RF sensitive stuff. Range is not exiting, and you need a power cord, but it is so cheap and easy... Don't put your body parts into the beam from horn, and don't aim it on good people even from a long distance. If you have hands in right place, you could make a portable EMP gun with batteries, DC-DC boost converter and some simple elctronic stuff to control magnetron power. However, batteries will not last long.

If you questioning if it is legal to build and use it to fight against authorities, elites, bigbrother, etc, then, I think, you don't really need any weapon at all, because it will be always 100% unlawful to resist authorities, elites, bigbrother, and all other entities who wrote that laws.