Along with the usual methods, pilot driver, hammer drill, we have developed the simplest alternative in a core driver. It is a hardened steel rod that goes down the inside of the post and the point sticks out about 1” at the bottom and it has a cap on the top.
You insert the driver, like loading a musket, and then pound the driver and post in with any means you wish. When finished pull the core driver out and insert in the next post. It makes a pilot hole and installs the post at the same time.