Use at your own risk for sure, chlorine gas is not to be messed with.My dad once said that he would do this, btw, I know it is concerning, but sill, efficient. He would pour bleach into the anthill and then pour ammonia into the hill too, then he would quickly cover the top with a bucket and leave it for a few hours, and presto, no more ants. If you use this method, use it at your own risk and keep the chickens away from the war zone for a month or so.
Fire ants may be resistant against water, but ya, boiling water can kill a lot. Here is the problem though, you will need a lot of water to kill their queen. Fire ants place their queens deep underground, deep enough that rainwater doesn't bother them, and to get rid of a colony, you must get rid of the queen. Also, I agree with the avoidance of using chems, I just wanted to tell you a possibility.
