i believe this is the sure fire way to get this one. i have spent over 10 hours no joke trying to get it on the London tower level but that failed. so here is what i did. load up act 2 chapter 5 its the D.C level the one where you go in the white house. progress through like normal pick up a non assault rifle one that shoots like a bullet at a time. progress like normal til you reach the inside of the white house building now start shooting at there nose. this is what i did i got it in 10 kills i believe or it did not feel like much more it was not 15 for sure. after 10 hours of work this part i get it my first try the key is shoot there nose they tend to make a scream if you shoot there nose too i hope this helps give it a shoot.
Edit: i did it on easy all cheats try and keep track restart if you dont think you got 10 after the 1st checkpoint in the white house