[MTScott2]

Use what you got...A last ditch effort is to get a small sharp chisel and wack a v groove into the edge of the head, then bang the silly offending fastener in the off direction... the fastener is buggered anyway. So have a new one on standby.
Use lots of construction talk some say it helps. S@#¥ f€@%%! Stuff like that

 

[MTScott2]

Being a retired 40 year pipefitter I've loosened an uncountable number of corroded nuts up to 6''+ , can't remember any that didn't come off (think torch).
A Very Very last ditch effort, would be a small 6 or 8 inch Rigid pipe wrench, adjusted just right so it bites deep before applying the needed torque, and a cheater pipe for pull, even a jack under to push up on the wrench sos it won't slip down and off. We call it vibration when you give it a wack with brass or steel whatever...
I used to put a 3/4 or 1 inch drive impact on stubborn fasteners and go full throttle for minutes at a time until it heated up enuff to break loose.
Builds character....