So here's the simplest way to get some easy creds in Reach. There is another, more repetitive way to get many more credits (which I will link to at the end), but it's really a one time deal and is EXTREMELY BORING.
What you need:
-The ability to easily headshot grunts with a DMR
What you will do:
-Enter "Score Attack" under the Cooperative section in Matchmaking. Although it's under Cooperative, you will be playing this mode alone.
-Choose "Gruntpocalypse." If it doesn't show up, just choose "None of the Above."
-Just keep killing those Grunts! You definitely wanna get headshots and go for double and triple kills since most of your points will be coming from Commendations. At the end, you'll get around +600 for Commendations, +300 for Game Completion, and whatever you get from Slot Machine.
-Keep in mind that this is a good way to get those "Kill XXX enemies in any game mode" challenges, but all challenges regarding Firefight Matchmaking won't work in here. Sorry guys.
Credit goes to TheTrevHD on Youtube and his video:
The "other" method, also courtasy of TheTrevHD: