You can kill a razorback boar using a Molotov cocktail once you reach the Ruins Encampment base camp in Geothermal Valley. There is a crate of bottles of alcohol near the campfire that you can use to craft the Molotov cocktail. Then, jump down to the area next to the river and ruined log cabin to find a pack of razorback boars to use it on to get the achievement.