Turok

This achievement is story related and will unlock once you defeat the T-Rex in the level "Mother Superior". As soon as the fight begins take out your knife so that you can run faster and head underneath the tree in the centre, now equip whatever gun you have available and shoot the T-Rex, it will take a while but eventually the T-Rex will collapse and you need to run towards it and stab the back of its head to trigger a short cut-scene and unlock the achievement.