A technique I've often used on tough plumbing or boiler connections is to put a heavy metal backup mass such as a heavy hammer or piece of steel against one flat of the nut and then smack the opposing flat with a hammer. This has the effect of momentarily ovaling the nut by a few thousandths to break the rust bond on the threads. For plumbing unions, usually a pair of big pipe wrenches works. For smaller nuts, I use a couple of hammers but you may need to use metal block (such as a bodywork dolly) and a large punch to get an accurate sharp blow, which may necessitate the use of a third hand. I have gotten creative with bungee cords or duct tape when no help was available. It's important to have as full and accurate contact on the flats as you can or you may just deform the nut permanently, plus you don't want to miss and smash the exposed threads of the bolt.You may need to go around several times on adjacent flats, and use more PB Blaster. If nothing else, this helps the penetrating oil penetrate. Don't try to smash the nut, you want it to spring back to round.