There is an Achievement for "Goliath, Meet David - Allowed a Goliath to level up four times before killing him" but I don't know how Goliaths work to figure out how to do this.
When I've come across them before, they seem to attack me until I behead them, then some weird shrunken head atop of stringy neck comes out and they start rushing towards their friends (my enemies) and attack them until there's none left.
So how and why does this happens, and what are the actual Goliath AI rules? How do they "level up"?