This achievement is obtained in ‘The Black Captain’ mission.
I found the best way to go about killing the Hammer is to use the Uruk’s who support him to build up your combo count. Once your combo counter turns red use a combat execution ( Y+B button together) on the Hammer. This will take a small chunk of health off the hammer. Then repeat the process again. As the Hammer slowly losses health he will bring in more uruk’s that you can use for your combo’s.
When the Hammer has no health left be ready for a QTE event that you would have already seen with warchiefs in this game. Push the pointer in the direction of the circle on screen and when it’s in the circle, press the indicated button (Y,X,A or B) to finish the Hammer.
You can use the Hammer to increase your combo’s but he has area attacks that are very good at breaking your combo’s, also dealing damage to you aswell. It’s best to avoid him until the execution part.
You can also use the bonus objective, for shanking Uruk’s (hold right bumper to grab then X to shank) to quickly increase your combo if you choose to.
Good luck with the achievement.