Wd-40 / Liquid Wrench / PB Blaster, let sit overnight.
If all else fails, but you'll incur some expense and you have to be careful...
But with an angle grinder shear off the dang bolt head, works every time
Just careful you don't grind into the spindle or the blade, also a blade bolt is like $10-$12 each.
I would try the solvents and a breaker bar and some patience first.