Pre Version 1.2.2:
Mechs spawn on higher difficulitys (High (I am not sure) or Insane (I am sure)) and respawn very quickly.
After you have found and killed one, just run/fly ~50 meters (~164 american feet) away and fly back.
Post Version 1.2.2:
They added a lot more enemys and I cant test anything, because I already got the achievement. I THINK you can just walz everything down and it will pop eventually.
PS: The name of this achievement is probably a reference to Metal Gear Rising! I love this :D