You'll need an Electro Gun , you can craft it with :
Harpoon Grip
Tesla Coil
Big Fork
Capacitor
Hardware
There are two Tesla Coils on the first Island of the DLC, Horno Island.
You can carry over the components from the main game to obtain one Tesla Coil from a Monkey on Horno Island. Bring a Reel, Red'Stone and a Short Tube.
The Capacitor is on the second Island, Cenizas Island. It's in one of the safe houses.
Credit to MastaGus for this introduction
Once you have the weapon you can reach the crowed area in the easier islands of the main game and start kill zombies, i recommend to use an Electric Jacket to keep the weapon loaded
as FFX Brotherhood sad, have the WEATHER REMOTE CONTROLLER (you get it from finishing El Diablo DLC) can help a lot !