Happy Tree Friends False Alarm

Select the first Ski Lodge level. The very first room of the level is the perfect place to burn them all to death, because they cannot leave unless you allow them to. Barbecue the 5 Friends for about 30 minutes and there you have it.

In the mine set of levels on the second mission 'From The Bottom of My Cart' the Tree Friends are stuck on a little walkway at the top of the level. Just burn them up there. Once you kill 4 of them the 5th will explode. Using this method you can get the achievement in around 30 minutes.