What's the best way to shave off 1/8 of an inch from hardie board?
You can use one of those cheese grater looking rasps most people use for drywall. They're called serrated contour planes.

Something like the Stanley Surform Pocket Plane or Stanley Surform Plane-Type

Those tools will be quicker/easier and make less of a mess than using a power saw. Or at least a more controlled mess. You can also use a utility knife with a sharp blade but the rasps are easier to use and safer.