After realizing that they are dealing with changelings, the Winchesters arm themselves with flamethrowers due to fire being the changeling's weakness. During the fight that follows, Sam is able to use his flamethrower to kill the Mother Changeling, destroying her children as well.
Upon learning that Jack Montgomery is a Rugaru, the Winchesters arm themselves once more with flamethrowers. Ultimately, Sam is forced to use his flamethrower to kill Jack when Jack is unable to control himself.
In order to interrogate a Crossroad Demon for Crowley's true name, Bobby Singer scorches her original human bones with a flamethrower, causing injury to the demon. Bobby breaks off his torture after being interrupted by Marcy Ward, but resumes once Marcy is gone. Finally, the demon gives up Crowley's real name and Bobby uses the flamethrower to burn her bones, killing the demon.
After killing Ezra, the angel Josiah was directed to a warehouse in Pray, Montana where the Heavenly Portal was supposedly located. However, Metatron had set a trap wherein holy oil was dumped on Josiah and then ignited by a flamethrower, fatally burning the angel. He died awhile later after rejecting Castiel's healing.
After showing up to help rescue the Winchesters, Sheriff Donna Hanscum asks if anyone knows how to use a flamethrower. Claire Novak later uses Donna's flamethrower to kill a monster from The Bad Place, but discards the weapon afterwards.