Metal: Hellsinger (Xbox One)

Behemoths are large, one armed melee creatures that you'll first encounter in the opening level. They can cause quite a lot of damage, but their attack is easy to dodge and they are vulnerable to ultimate attacks. In later levels you will encounter a more powerful variety called Siege Behemoths, which also count towards this achievement.

This is one of the achievements you'll almost certainly get naturally in the final levels, so don't worry about trying to grind it out. If however you want to get it earlier, after you complete Stygia you'll have access to the Giant Slayer Torment, which requires you to kill 10 of them. So you could just keep playing this until you get it.