I thought I replied to this, but kroil is life. PB blaster if I am poor. I also use heat cycling, heat making things liquid, intentionally breaking bolts then drilling them, nut cutters, Impact wrenches, impact of a hammer beating on a cheap wrench, cheater bars, more hammers, air hammers, etc. I feel like the Bubba Gump of breaking stuff. Gonna win though.
Something that goes a long way though is maintenance. A thread gets anti-seize, pipe tape and/or dope, or rarely loctite or specialty lubes like moly. Never dry. Tapered bearing surfaces get lube too. Never dry. An engineer that specifies a dry torque is lazy and dumb since the friction is too variable and also not what holds a thread.