For this achievement you need to find a shotgun and shoot Jason with it. The shotgun is found semi-randomly inside buildings and campfires and is easy to spot.
If you're boosting this game (and you probably should if you want to get all achievements), I recommend using the map "Crystal Lake Small". There's a building that has tools in its icon and has the 4-seat car outside. Inside this building there's always a shotgun in the far left corner (when you enter through the garage door).
Alternatively, there's also an achievement for playing as Tommy Jarvis 13 times, he always spawns with a shotgun so you can kill 3 birds with one stone as he also spawns in with a pocket knife and there's an achievement for using that 13 times too.