This achievement can be earned online, or against bots
From the main menu:
- CREATE ROOM
- Switch mode to Free For All
- Put the Time Limit bar all the way to the right, making it 10
- Put the Max Players bar all the way to the right, making it 10
*Make sure Allow Bots is checked, so that bots will spawn in your game
- Check off every map EXCEPT Countryside
The Sniperagus Rifle is the sniper in this game, and it has 6 bullets in its mag. This means you can miss two shots and still get the achievement.
Every headshot with this weapon is a 1 hit kill, no matter what the health is of the enemy.
At the beginning of the game, every gun spawns with full ammo in its mag, so go straight for it. I would recommend picking up the milk cartoon in the top of the barn to get 100 extra health.
From there, this is just completely skilled based. I can't really offer much more help other than just practice. You probably won't do it first attempt, so practice will happen automatically upon trying!