Crackshot -  3000cR 

Kill 70 enemies with sniper rifle headshots in Nightfall on Heroic.
 
You can simply play the very first sequence where you kill a stationary elite over and over to unlock this.  If you wanted to just reload this 70 times you could probably unlock this in under 5 minutes.  
 
Yesterday, I got a handful of headshots on this mission, then did a Save & Quit to see if it would save my progress.  It did not (reverted me back to 0/70), so I reloaded the mission and just played the first section a bunch of times (reverting to last save) to unlock this challenge in one play.  I was disappointed that you couldn't spread this over multiple plays or I probably would have seen how many headshots I could get just playing normally once, planning to go back and clean up the rest with a second play if necessary.
 
IMO weekly challenges should be fun (for at least a healthy % of the player population) to work towards and take longer than 5 minutes to complete.  Ideally it should be something that would take at least two 1-2 hour play sessions to complete.  But that's just my opinion.