en_tn/jdg/09/52.md

fought against it

"attacked it"

upper millstone

Two large, flat, round stones were used to grind grain in a mill. An upper millstone was the top one that was rolled on the lower one to crush the grain in between them.

armor-bearer

This is the man who carried the weapons of Abimelech.

pierced him through

This means the young man put the sword right through the body of Abimelech.