Single Player -> Slaughterhause -> Select Redneck class and do as I did on the screenshot below:

You can put a tick on Enemy Hp bar (not sure if it's ticked by default).

Cooldown time = 0 (just few secs)
Difficulty = 0 (easiest)
Wave size = max (40-ish enemies per wave)
Number of waves = this doesn't matter much* but you can set it to max as well

* - It doesn't matter because when you beat wave 1 or while you're still at it it will be night time already so it's better to just restart the game.

When you start the game look for weapons. There should be a few at a place (they will be marked with yellow outline). Pick up ak47.










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Look for any door marked with red outline. Enter it by staying in front of it and pressing "E" button when the hand cursor appears.

✓ - Play only while it's day time (optional but recommended)
✓ - Play versus faster moving animals (like stags / foxes). If you start the mission and enemies don't spawn in 1 min or less just click restart. Some animals (like giraffes) are really slow so you will get achievements at a slower rate too.
✓ - Use stronger weapons (such as Ak47)

✗- DO NOT play till the end of the game (a.k.a. fight bosses as they only award you with 1 achievement when you kill them and are much harder to kill)