The Super Tyrants are the red clawed Tyrants you see at the end of The Places We're Meant to Die. The key thing to note is that the standard Tyrants, the Mr. X guys, will turn into a Super Tyrant if they're hit by the rail gun that your Spec Ops commander has air dropped through the roof for you. Once you are facing a Super Tyrant, draw him away from the rail gun while it recharges, and make sure he is not in line of sight when it is charged or otherwise it will hit him again and kill him, which doesn't count for the achievement. Instead, draw him around a corner and kill him with your guns and grenades by shooting at the red spot on his chest. Once he's down, draw another Tyrant to the rail gun to be transformed, drawn away and killed by you and your team. Repeat one more time for the achievement.