If you have lots of time to spare, try this challenge.

Level: truth and reconciliation
Difficulty: Heroic or higher.
Skulls: "Boom skull" "Grunt Funeral skull"
Weapons: Keep your sniper rifle the entire level

Tips: Don't take grunts lightly. If there are a bunch of them kill one, Get to cover and then wait for the rest to be killed
this challenge may be easier at first.. But when you get to the point of having Captain Keys in your group you will begin to fail alot more.

Extra tips: Captain Keys can die a lot easier than you, when he dies you fail the mission and then get restarted at a checkpoint.

Good luck!

Reason for edit: Grammar.