Here's a little tip for 'Cold Blooded' challenges (if you want to go past just getting 'pro').
If you ever see a helicopter spinning out of control, you can just shoot at it with your primary weapon and every bullet that hits, counts as a cold blooded kill (assuming you have cold-blooded on ofc)... you don't see the 'hit detection' indicator, but as long as you're on target, it will work
The problem is you won't see many, as Stingers/Javelins will kill helicopters in one shot (they just blow up)... but if you use RPG/AT launchers, they apparently will never blow it up, just cause it to spin out of control when it runs out of health.