Aliens vs. Predator

The best way I found and the way I used when I got the achievement was when you have to kill weyland, you take out you machine gun and shoot all 4 of your m2o3 grenade launcher bullets in to him. He should be dead, but if not just shoot him a bit with the machinegun. After you kill him, go get the data pad and when weyland throws you down the stairs press left trigger to 3 shot into his head.

I have not played this on nightmare yet, but when you get to this part I would (am Im going to try this) is once you are done with the grenades you switch to a shotgun and pull left trigger to shoot both shots at same time, reload and repeat till he is dead.
then follow same as above.