I WAS HEARING Remy and completely understood his question. Thus my answer, the only way I thought it was possible to kill WITH world was using the bomb.
But maybe it's the airborne thing. If you shoot/nade them in midair and they hit the ground with under 10 or 5hp and it kills them...you get a "world" kill?
Now I'm definately curious, b/c I was pretty certain it was the bomb, but apparently not.