So this is a glitch I discovered from being lazy, this glitch is for those that say: complete x amount of missions
What you need:
2 Save devices (Hard Drive, flash drive, mem card, etc)
1 ) Prolly the first mission is easiest, get to the very end of the level, right before you do your last lil thing to go to the next level (like on the first level, before you press the button), save and quit.
2 ) Go to your dashboard, My Xbox -> Settings -> Memory -> the device you were NOT playing on -> Games -> Halo Reach
3 ) IF THERE IS A SAVE that has a title of your gamertag, delete it, this is old campaign data
3 ) Go back and go into device you WERE playing on in Reach, go to the Reach folder and copy the campaign save to your opposite device
4 ) Launch up Reach, select one of the 2 memory devices and hit resume campaign
5 ) Finish the level, go to the Dashboard -> My Xbox -> Settings -> Memory -> device you WERE playing on
6 ) Delete campaign data
7 ) Go to the device that you WEREN’T playing on, copy the save to the opposite device
8 ) Goto 4 until challenge done
Yupyupyup, that easy
Basically, you’re just backing up the campaign save, finishing the level, and restoring that campaign save to do it again.