Get all your holes started to the point that you hit the clay. Then take a rock bar, or some other item that can penetrate a few inches into the clay, and poke a few holes in the clay. then take a half bucket of water, or so, and dump it in the hole. Wait a little while, then the clay will be much easier to work with.
When I put in my fence we hit clay so hard and dry that it sheered off my auger bit from the bolt that pinned it on. Sheared through 1/8" steel. nasty stuff.