For the Danger, High Voltage achievement, you need to stun four law enforcers by shooting a Zapper's battery pack on Hard difficulty or above.
A great opportunity to do this is during the Dirty Ice heist. Wait for the heist to go loud, and look for a Zapper to come through the corridor between the manager's office and the VIP Showroom. Often, you'll find a group of cops already in pursuit. Your target is the Zapper's battery pack. When you shoot it, the resulting electrical discharge will chain between the enemies, stunning them.
It's important to note that the timing and positioning are key here. You want to ensure that there are enough law enforcers close to the Zapper to maximize the effect of the electrical discharge. This achievement requires a bit of luck and good timing, but with the right setup, you can earn it relatively easily.