There are many special achievements in "Lake's Servant," and the Pest Control achievement is one of them. To earn this achievement, you need to go to the [Inspect Cellar] level, select the sledgehammer and magnet items first, and then head to the area with the mouse hole (choose the door on the left at the fork).
How to Get the One Hit Mouse Kill Achievement in Rusty Lake's Servant
Pest Control
One Hit Mouse Kill
In the [Inspect Cellar] level, select the sledgehammer and magnet items first, then head to the area with the mouse hole (choose the door on the left at the fork).
Interact with the hole using the magnet to pull out the key, then click on the hole again to make a mouse crawl out.
Use the sledgehammer to kill the mouse as it runs from the left side of the screen to the right.
Related Guide: How to Get the Garbage Disposal Achievement in "Lake's Servant"