As stated by the above guides, this achievement is for killing 3 Tyrants.
But there's another option to get this achievement aside doing chapter 5 on campaign over and over again.
You can also fight a Tyrant in Versus mode, Team Attack, on the map "End of Days" after a certain amount of time (around half of the match).
So if you really don't want to make a second playthrough, you can get your achievement there.
Have fun :)