To prevent the flamethrowers from working you need from the first soldier who uses flamethrowers and you will not kill him, shoot him simultaneously (if you are in brutal he will not die so easily) that way he will run following you . Keep running and let all the flamethrower type soldiers live and when you have 3 running after you the game will simply not be able to load soldiers using the flamethrowers anymore because you have too many soldiers of this type running following you. I hope it works for you