The Creature Lives Achievement in God Mode

To enrage a Cyclops, you need to hit him in his eye. It may be quite difficult to hit his eye, but you will know if you do because he will either cover his eye with his hand and then swing his club around like a madman or just act unstable in general.

If you are unsure what a Cyclops is or what it looks like, you can visit the bestiary in the main menu and see one.