To get this achievement, you need to knock someone into an obstacle that is on fire like a barbecue grill or brazier. The easiest place to do this is on the Goat City Bay map. On this map, there is an NPC named Marie that is normally standing next to a barbecue grill.
To find her, go to the Cabrito amusement park and find the small, sandy beach under the roller coaster. Once there, find the barbecue grill next to one of the roller coaster's support beams. Marie is in jeans and a yellow t-shirt. She will path between laying down and standing next to the grill. Once she is near the grill, head-butt or kick her into it. If she accidentally trips or falls, you can still kick her body into the grill.