I never understood the resistance to using wrenches that actually fit and a real barrel vice. Cooking the barrel will ruin the patina and case hardening if present on the plug. It is not big deal to make a properly fitting wrench. Since I work with both modern and ML guns, I never give it a second thought. Lots of destructive ideas being recommended here. IF the gun has any value I'd recommend more thinking and less brute force.
I re-read the OP. To back peddle a bit. I guess it you are going to throw the plug away, do whatever you want. An excellent way to loosen scrap barrels on 1917 Rifles is to turn the shoulder off the barrel in a lathe. Turning the shoulder off the plug with a parting tool will make it unscrew with no resistance. Cut up close to the joint between the barrel and plug. No shoulder, no tension, depending on how rusted the threads are.