I am only missing the Full Recovery challenge which is to kill 5 enemies before getting healed.  I figure I should set the difficulty to rookie, but what game mode and maps should I use to attain it?

Alright, load up a Co-Op Hunter on Yastreb Complex - Rookie difficulty. Have your co-op partner lie low the whole time. Let him just chill in the tunnel where you spawn while we get you that challenge. Get out of the tunnel and hop on top of the train cart. Shoot some rounds and attract a lot of attention and reinforcements will be alarmed and they'll swarm in the room. Its a really wide open room so  they won't have too many walls or things to hide behind especially if you have the high ground on top of the train cart. Let them gun you down while you're on top of the train cart. Sit up and shoot away. You got a good 60 seconds before you bleed out and for the sake of it being on Rookie difficulty they aren't going to take very man shots at you while you're in this state. Kill the 5 guys in a few seconds and enjoy your points. Repeat for your partner if he wants to finish the challenge too.

" Alright, load up a Co-Op Hunter on Yastreb Complex - Rookie difficulty. Have your co-op partner lie low the whole time. Let him just chill in the tunnel where you spawn while we get you that challenge. Get out of the tunnel and hop on top of the train cart. Shoot some rounds and attract a lot of attention and reinforcements will be alarmed and they'll swarm in the room. Its a really wide open room so  they won't have too many walls or things to hide behind especially if you have the high ground on top of the train cart. Let them gun you down while you're on top of the train cart. Sit up and shoot away. You got a good 60 seconds before you bleed out and for the sake of it being on Rookie difficulty they aren't going to take very man shots at you while you're in this state. Kill the 5 guys in a few seconds and enjoy your points. Repeat for your partner if he wants to finish the challenge too. "