For this achievement you need to kill someone in a game of 'Hillcampers' when they are 1 point from winning. I found that the player
does not need to still be in the hill so you can simply have the person not going for the achievement run into the hill until they are 1 point away and then run out of it, then with the player going for the achievement kill them.
The settings I used in a 2 controller local match are:
• Map - Any (I used Museum)
• Gamemode - Hillcampers
• Gamemode settings - Score to win 5
• All other settings were default.
Here's a video using 2 controllers.