I watched the video and I don't think I saw you get one kill where you weren't right next to your high level friend also shooting at the enemy. It looks like you are just getting the lucky last shot in and getting credit for the kill. Finally at around 3:50 you do seem to try and take on a guy looking down out of the building, you hold down the trigger for about 2-3 seconds and you take out his shield and a little bit of his health less than 25% and then you were one shot with a shotgun. At 4:02 you unload your auto-rifle clip for about 4 seconds and you take him down to about 25% health. There does seem to be a difference as it only takes about 1-2 seconds normally to kill someone in crucible with an auto-rifle. It doesn't seem that you can one shot low levels like you might have been expecting but there is a difference.