Using Craftsman mechanic grade 13mm socket and wrench. I've never had a problem like this before either in 40+ years. The car does have 106,000 miles on it. I presoaked them all in WD40 and even tried heating with a blow torch and alternating heat and WD40. The heads just rounded off. Then I resorted to vice-grips and a hammer because there was nothing left for the socket to grip onto. Nothing!
Good stuff, use it all the time. Spray the hell out of the bolts let it sit, spray some more let that sit! You get the idea. May take overnight to be effective. The 12mm may be too small, 1/2 is just a smidge smaller than the 13mm. Always use six point sockets!