I found this specific achievement to be immensely bland. I felt that restricting this achievement to the campaign was a bad move when you have a perfectly viable avenue to continue exploring the game through custom maps from Steam Workshop.

Unlike the sequel Heroes of Hammerwatch, this game's campaign has a lot less RNG involved and it's just a straight-up snooze fest after the 4th or 5th time playing through the mode. I wasn't particularly enthusiastic about replaying the campaign just to get to a set of spawners to farm for the kills.

When you have Steam Workshop available which provides infinite possibilities to add replay value, why would you restrict the achievement to the mode which has less (replay value)? I understand the method described below will be controversial to some, but I also feel just as strongly about bland achievement design.

I extracted the resources from the campaign.hwm and repacked it with bat spawners near the very beginning. Normally if you repaste the campaign.hwm in the correct folder and then launch the game, you will be unable to start the campaign (the button won't work).

Note: If you try to paste a completely different map. You can end up with a completely black screen/map when you launch the campaign.

To bypass this, I kept the game OPEN while replacing the campaign.hwm and then hit the start Castle Hammerwatch button. This will allow you to use the modified campaign until you restart the game.

Steps:

1. Download the modded campaign with bats[drive.google.com]
   
2. Rename it to campaign.hwm
   
3. Launch the game.
   
4. While the game is running, replace the original campaign.hwm inside
   C:\Program Files (x86)\Steam\steamapps\common\Hammerwatch\levels
   
5. Start the Castle Hammerwatch campaign.
   
6. Walk right to discover the bat spawners close to the starting area.
   
7. Set yourself up at an ideal spot (left, right, or top right of the spawners) then start up your auto-clicker (or other preferred method) to run for 13-14 hours.