“Who is the mightiest of the mighty? He who makes his foe his friend” (Avot de Rabbi Natan, Chapter 23).
In ethics, we should interpret that “mighty” is “one who conquers his inclination” (Avot, Chapter 4). That is, he works with the good inclination and subdues the evil inclination.
The mightiest of the mighty is one who works also with the evil inclination, as our sages said, “With all your heart—with both your inclinations” (Berachot 54), where the evil inclination, too, serves the Creator. It follows that he makes his foe, the evil inclination, his friend. And since the evil inclination is also serving the Creator, it follows that here he has more work, for which he is called “the mightiest of the mighty.”