4e doesn't have rules for aiming at weak spots, beyond the use of relevant powers. Which would certainly help in a skill challenge to minion-ise the dragon!While that sounds like a good idea, it does not need minionizing. You can attribute the instant kill on the black arrow and vulnarability in that certain spot.
Question is: how do you treat the attack roll? Is it harder to aim on tge weak spot? Seems totally anticlimatic to win the skill challenge just to miss with the arrow...
Help action? Inspiration? Bard*ic inspiration?
Or just leacer the to hit roll out? This would turn the dragon fight in just a story.
I don't know of any other process in 4e for achieving an instant kill against an Ancient Dragon.