I've seen many threads that the achievement is very tough to get. Many people try various tactics, but it usually only works spontaneously. Personally, I've tried many tactics I read about as well and none of them did the trick. But getting this achievement is extremely easy if you know what you should actually be doing. I've done an extensive research on how it should be done, and here's me sharing this with everyone else.

Important note here, I'm writing about version 1.2.0, which is the latest on Steam at the moment of writing.

So, without further ado. The Steam achievement reads like this: "Hunt 20 deer before the end of day four." Here's how it should be written correctly:

Get the 10th deer kill on day four.

Yes, you've read this correctly. It should be 10 deers, not 20. And you should get your 10th deer kill exactly on day 4. If you get it earlier, it won't unlock.

Once you know this, the achievement is quite trivial: just hire several archers and get a couple of deers each day for the first three days. On the fourth day, just start herding deers non-stop until the achievement unlocks. This will happen during the day right after you kill the 10th deer, so you don't have to wait for anything. Of course, you can get 9 deers on the first three days and only get the last one on the 4th, but it's a bit risky because an archer may kill a deer by himself when it's not the time yet.

That's it. It has been verified, it works. Hope this saves a ton of frustration for the other players when playing this otherwise amazing game.