Disclaimer: Completing this new challenge in addition to the original doesn't give you 2 entries or tickets into the contest.
EDIT: I was wrong, here's a quote of what greg said.
There was a communication error with Microsoft regarding this challenge, and we were apparently not supposed to use the image of Master Chief within the game. We were asked to remove the bonus level immediately.
Since the challenge requires finishing this level to earn the ticket, the challenge task was therefore changed to 30 kills so that people can still earn the challenge if they were putting it off until today. You cannot earn the ticket twice if you’ve already earned it.
I apologize for the confusion regarding this matter, but it was not changed due to player feedback.
Edit 2: Challenge extended for 4 more days.
The original challenge is also back.