Without relying on Lucky, that is.
Among all of the special weapons available in Sker Ritual, the Thunderbolt is the only one that players can access on every map. This is thanks to its inclusion in the list of weapons that Lucky, the game's mystery box feature, can grant.
However, on the map The Cursed Lands Of Lavernock, the thunderbolt can be acquired through alternative means. You are required to use it if you want to finish the map's story mission, too. Thankfully, the thunderbolt is one of the best weapons in the game. As a result, you should immediately unlock it if you want the best chance of beating the map's story.
Destroy The Generators
After purchasing the first two doors on your immediate right from where you spawn in the chapel, you'll enter the courtyard of a small village. Here, the game will prompt you to destroy four generators hidden within the village. You can shoot the generators, of course, but it's quickest to melee them if you're going for speed.
The first generator is on your immediate right after opening the door that leads into the village. It's very easy to walk right past it. After purchasing the door, turn right into the corner of the room, and it will be right in front of you. The second and third generators are located to the left and right of the well at the center of the village.
All of the generators have big tubes on the ground that lead to them, so if you get confused, follow a tube line until it leads you to one.
Finally, the fourth generator can be found on the second-floor balcony of the one building in the village with a second floor.
Open The Laboratory
Once all four generators are destroyed, the electric fence over the large gate at the back of the village will go down. This allows you to purchase and unlock the gate. Just beyond the gate, you'll find the entrance to a mausoleum. Unlock that door, and you can descend into the laboratory below.
Open The Main Chamber
The laboratory is an extensive network of rooms connected by lengthy hallways and tunnels. Thankfully, the door to the main chamber is directly accessible after unlocking the mausoleum door. Head underground, take a left at the police purchasing station, and follow up with a right at the next opportunity. You'll wind down a short hallway that leads to the main chamber door.
You'll likely need to kill some more zombies to afford to open the door. The end of this hall is a great place to do it, as your back is protected from any zombie spawns. Once you've got enough money to unlock the main chamber, you'll find another police purchasing station inside, as well as some perks and the stand for the game's pack-a-punch mechanic.
Unlocking this chamber also progresses the story far enough for you to begin seeking cylinders to unlock the vault door. To unlock the thunderbolt as quickly as possible, we'll want to head back the way we came up and out of the mausoleum entrance once more.
Four waypoints will pop up on your screen once the main chamber is unlocked. Once you start a cylinder quest, you won't be able to access other quests until the current one is complete. Ignore the other waypoints and head to the one detailed below for immediate access to the thunderbolt.
Enter The Corn Fields
Upon exiting the laboratory through the mausoleum entrance, you'll head back to the village. Instead of entering the village, turn to the right, and you'll see a thick cornfield populated with several zombie scarecrows. Walk into the corn fields, and at the center, you'll find a chest before a huge tree with gnarled branches. Interact with the chest to start the quest.
Kill The Banshees
Upon opening the chest, five banshees will spawn in the corn fields around you and immediately attack. The banshee is a fast, teleporting zombie with a white sack on its head and a terrifying scream. They can be annoying to kill due to their teleport, but with enough bullets, you'll mow them down soon enough.
Wait until the end of a zombie round and leave one zombie remaining before opening the chest. This allows you to deal with the banshees without having to worry about the regular horde of zombies each round throws at you.
Purchase The Thunderbolt
Once all five banshees are defeated, the thunderbolt will become purchasable from the chest you previously opened. It costs 2,000 pounds, so be sure to save some of your currency instead of impulse buying everything in the laboratory's main chamber.
You'll need to use the thunderbolt to finish the rest of this quest, but its real value comes from using it like any other weapon in the game. The thunderbolt has a chain lightning feature that causes its bullets to deal damage to other zombies near your target. Consequently, it's one of the best weapons for clearing waves of zombies simultaneously.
The thunderbolt is best when you "train" zombies behind you, grouping them up in masses. Run a circuit across a small portion of the map to gather the zombies together, then knock them all down with a few well-placed lightning shots.
This weapon can also headshot, so keep aiming for zombie heads with your new lightning launcher. Once you've upgraded the Thunderbolt to tier 1, the difficulty of this map becomes trivial in comparison to the other episodes. In other words, sit back, relax, and enjoy that sweet zombie-killing buzz as you cakewalk your way to the end of this map's story.