This is an optional part during a story boss. On the first part of The Salvation, you fight the Terror Walker and before you defeat it, you ride it like a Nightsister on a Rancor. To kill it, you put it into the shield thing you have been working on to annihilate it's sheild. DON'T DO THAT IF YOU WANT THE ACHIEVEMENT. Instead, there will be Terror Biodroid (the ones with the face a General Grevious) and you want to kill all of those. Attacks for the Terror Walker are kind of difficult to aim. Find a prefrence and kill those droids to get the achievement.