You need to start the Lord of the Hunt campaign, and complete the first mission in order to get this achievement. In the first mission, you'll have to reach the Uruk camp marked with the yellow dot. Once there, you have to eliminate the five Uruk hunters, also marked with the yellow dots. Additionally, if you take out seven Uruks in total with stealth kill (
+
) while riding the Caragath, you'll get the bonus.
Once the Uruk hunters are eliminated, you will be instructed to return to Torvin. Once you return, a cut scene plays and the achievement pops.