Borderlands 4 Whack-A-Thresher Mission very likely takes some inspiration (if not all) from Tremors, a 1990 classic thriller featuring giant worms. Of course, with the storyline being set on Kairos, this could all be canon to the planet and not a mere coincidence.
Coming back to the Borderlands 4 Whack-A-Thresher Mission, it's not too hard to complete, and it's very low-risk as well. It can be completed in under five minutes if you know what to do, and the reward will be worth your time. That said, here's how to go about it.
Borderlands 4 Whack-A-Thresher Step-By-Step Walkthrough

To start the Mission, you will have to travel to The Howl, an area located in Fadefields. Next, you'll need to find Mort and talk to him about his problem. Make sure to complete the "Breeding Daises" Side Mission first, or this one will not show up.
After you interact with him, you'll be tasked with wacking Thresher (giant worms), which means you will have to do ground slams. This is easy, but can be tricky since they move about and only periodically pop their heads out of the ground. You can't shoot them either.

To overcome this issue, there are several Grapple points that you can make use of to get some airtime. Jumping is also an option, if you can time it perfectly. After you whack the Thresher enough times by slamming down on the marked holes, the Side Mission will be completed. All that's left to do is return to Mort for your just rewards.
Rewards from Borderlands 4Whack-A-Thresher Side Mission

Once you've completed the Borderlands 4 Whack-A-Thresher Side Mission, you will be rewarded with the following:
- XP
- Eridium (x2)
- Cash (x1000)
It's not a lot, but since the task is fairly straightforward, the reward is justified. If you want something more challenging, you can always liberate the Order Silo in the region
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