Open your wardrobe, select the effects tab at the top (the one with a Habbo and a plus sign next to it, the last one). Select the Torch and then select Save Changes.
Place a Stack Tile somewhere in the room. Open Builder’s Club and navigate to Vikings and place as many Viking Coties as you need on top of the Stack Tile. To max out the achievement you need to burn 600, but if you want to place so many at a time and not a while 600, that is also fine.
Stand next to the Viking Coties and click on them 5 times each to fully burn them down. Your achievement count will go up 1 point per Cotie that is burnt down. Keep doing this until your achievement has been maxed out!