Heres my personal favorite challenge story.
I loved the cruelty one where you kill a guy, pick up his gun, and kill him again with his own gun.
It sounded like an impossible challenge at first.
But i was on a level, overgrown of all maps, where a last prestige rank guy with a golden Shotgun (yes on overgrown) kept running over the bridge side into and around the buildings and getting quite a lot of kills, he was good.
Anyways since into half the match i figured out his routine so i manage to kill him and pick up his obviously golden shotty and waited for him to return.
But either he kills me or i die from someone else for the first 3-4 times i tired until finally i was able to get him with his own gun.
It was kinda a pain and kinda exciting running around killing him, then looking for his gun then, trying to survive to locate him, and go Shotty vs Shotty with him the more experienced one with the gun.
By FAR the most satisfyingly challenge for me.