many many litres of elbow grease. If the pitting is too deep it will simply be impossible to remove all of it. What you might have to do is take the swingarm out of the bike completely and use some fine wet'n'dry paper on a random orbit sander (also could use a mouse sander).
Then go to a buffing wheel with good cutting compound.
Then go to a polishing wheel with a good polish compund.
That pitting is actual physical damage to the aluminium.........is gonna take some hours to get rid of it.