If you have Cold Blooded, and get it to pro (theres a glitch for this btw) then your name doesnt appear in red when an enemy drags his sights over you.
and i never look for names – always id the bad guys by their uniforms. lag can prevent a friendly player's name appearing especially at long distances so you may end up shooting friendlies if you shoot those with no name above them.
Smudger
SPOILER below…..
get Cold Blooded. get an AT4 rocket launcher as your backup weapon, use an LMG as main weapon. when the enemy get a chopper in the air, ONE team member hits it with the AT4, then everyone opens up on it with bullets. this prevents it from exploding but it will smoke, burst into flames and then spin to the ground for 5-10 secoonds. If you shoot it once it is dead, but spinning, your hits count as a 'kill' towards the Cold Blooded challenge. You wont get a 'hit' indicator, but so long as you keep shooting, the kills rack up. (you need 150 to get the Pro) The LMG can almost discharge its whole mag if you time it right and you can get the challenge in 2-3 goes if you do it right.