I'd have the frame on its side, fixed cup down and for me near the washing machine. Every time I walked by I'd add more liquid wrench for a week. It'll move easier, penetrating oils do work. Next I'd grab a quality wrench, not a thin park of bicycle specific, something like a Wright (my personal choice), SK, Proto or Williams; the kind that never slip. I'd use a bolt and 2 giant washers to hold the wrench aligned for extra measure and a frame alignment tool on the arm of the wrench for extra leverage. A quality wrench like wright will rip the head off a grade 8 bolt before it ever slips and they don't have much give in them. Don't use an adjustable it will fail as will cheaper wrenches.