Simply start a new game on the level "Reunions" watch the cut-scene then stab the first enemy. Proceed up the rock to your left and across the broken tree to another small rock. Wait for an enemy soldier to check the ground and launch your flare. Once it sticks, two dinos from the cliff to your left will attack him. After the dinos have killed him,(Wait a couple of seconds just to make sure.) pause and save your game. Overwrite. Then restart to your last checkpoint, and repeat 5 times for your achievement. I apologize, I was appearing Busy at the time so the achievement didn't show itself unlocking. Don't forget you can check your progress on the main menu!
I made this video a while back if you need more help.