Two, eight foot long 5/8" copper clad steel rods - plus - One 1 1/2" Milwaukee rotary hammer drill (no bit) - equals - Five minutes total to drive them both through clay and rock
For the last few inches I put an old 6" long 1/2" drive impact extension in the drill to use as a bit. Worked great.
I can't imagine how cave men drove their ground rods...