Use PB Blaster. Let it sit a few hours, whack the nits/bolts with a hammer. PB blast again then try to remove the fasteners in the morning.
Heat works wonders. Just be carful you don't melt anything.
I like to heat them up till they start to glow then Let them cool down completely and hit them with the impact gun. If they don't move after that I just cut/grind them off.
I usually just buy new hardware and cut the old off. Less time and helps get a lil destruction out of my system.
If your socket slips off, then you have rounded the nuts, and/or using a socket that's too large. If the nuts are rounded, then hammer a slightly smaller socket on them and hope you can get them off. If that doesn't work, then you will most likely need to drill them out (not fun).
Invest in a good cordless impact wrench (screw air, unless you want to fiddle with compressors and air lines), to avoid these situations again. They are way less likely to round off a bolt/nut, and have more torque than you can usually apply yourself.
PB Blaster is your best friend in those situations. I used to work in power plant maintenance and you wouldn't believe the stuff it would break loose. Definitely made me a believer