@CloudyMrs You might try removing the whole arm, soaking it in WD40 and then putting it into a vice (with padding to prevent damage) before applying a wrench on the now unmovable parts a decent angle.
A little too late now, but I always put a lithium grease onto the pedal axle before fitting them from new. There are two different metals involved (probably steel and aluminium) and corrosion has a horrible tendency to do this.
Good luck.