Complete the game on Hard Difficulty
Beating the game on Hard will also unlock Bond Broken along with it.

The game is arguably more challenging on Hard difficulty than in previous God of War games, mostly due to the multiple elemental mechanics and the drastic change to parry.

For additional tips for the more difficult bosses in the game please visit this post of the guide. For the Trial of Archimedes walkthrough, which for many will be the hardest section, please view this thread.