Check this video out - in the description you'll see where exactly I got the Phantom mark. My friend Casio and me both ended up getting a 25 kill streak - and I sure wasn't camping at all. Should have gotten 2 or even 3 consecutive Phantom marks, but that's not possible in one life. You'll get the mark after 7 kills without taking too much damage (if your health stays white and doesn't go into the red third).