Straightforward, once you see an electron mace. I first saw one on co-op mission 4. The Electron Mace fires a beam (looks kind of like lightning) which stuns and locks up one of your teammates. Kill whoever's holding it (usually a Sergeant) and this pops up right away.
These are most common in the Aspari missions, so look for Electron Mace enemies in Colorado, China, and Scandinavia (4-6). Chances are you're going to get maced more than enough in China, especially in the tunnels right before the final boss fight, so that's a good place to try and get this.
They appear on all difficulty levels, so you can get this on Normal, Hard, or Expert. Easiest on Normal, if only because the enemies holding the electron mace die the fastest, but not that much more difficult on Hard/Expert.