All three of these take about two hours, so I can adjust for how much time I have to play. You can also adjust for your own skill level.
Matchmaking Multiplayer, 250 medals, 3hrs: 10,622 cr
Matchmaking Multiplayer, 150 kills, 3hrs: 6,373 cr (recommend upping to 175)
Matchmaking Multiplayer, 5 wins, 3hrs: 2,125 cr (I’ve done as high as nine in 3 hrs, recommend upping to 7 wins)
Total 19,120 (and all the credits you got in the 10-20 games you played to get the challenge done, so add 10-25K)
I am very happy with these challenges because its not like you just run into firefight to get a bunch credits. You got to run into matchmaking and earn them fair and square. That is why I use these custom challenges, also because of its awesome payout.