This will start in the halo reach main menu.

1. Go to matchmaking
2. Scroll down to Cooperative
3. Click on score attack....not firefight! it does make a difference!
4. when qued up it will give you 3 different types of scoreattacks to chose from
a. Regular scoreattack
b. crash site
c. and gruntpocalypse
5. You will want to click on gruntpocalypse.
6. When choosing your class i suggest you use the armorlock class.
My reason being is that the skull catch is on wich makes grunts through ALOT of plasma gernades and armorlock will save your life alot!
7. When your game is done you usually get around 1,000-2000 credits a game depending how well you played, if you got any acomadations, and the types of kills you get (headshots, stickys, ect.)
8. Good luck racking up cR!

Tips!
[*] When playing always camp near ammo crates
[*] Use armor lock ability as a class...it will save your life!
[*] Get headshots on the grunts!
[*] always take out the higher ranking grunts...this will make the rest of the grunts scatter in fear. Muahahahahah.
[*] Champion grunts ( the white ones with helmets) take two shots to kill, one to take the helmet off and other to finish him.
[*] You have only 10 lives...use them wisely!
[*] Watch your back! Grunts tend to be sneaky >.<
[*] Finish off with extra time! There is a time bonus!

Btw this will get boring after a while so another great cR gaining thing is to play campaign on heroic with some gold skulls on!

I take no credit for this tutorial. Hope you like it.