If, like me, you're playing only in coop, the best level is 2-3.
One player fill the bucket and put it on the counter, then the other pick it up, empty it in the furnace and put it back.
Every 30 seconds, the level changes and players have to switch roles.
With this method, my partner and I could burn more than 60 buckets of coal (20% of the achievement) in one session. The level is 5 minutes long, so it will take you about half an hour to both unlock the achievement.
You'll see that the host will progress faster because of the layout, so do not hesitate to switch the host after 2 or 3 sessions.