How to beat this game on Easy and get the Medium & Hard achievements
*You will need TWO 360 controllers and a backup account signed in to do this trick.
Step 1 - On your MAIN PROFILE/first controller (the one you want the 1000gs to go to), beat the entire game on Easy. On this difficulty it will only take you 4 hours or less
Step 2 - After you beat the game go ALL THE WAY OUT to the main title screen. With the BACKUP PROFILE/second controller start a NEW GAME on HARD, choose Co-Op, and join with your Main Profile/first controller when the game screen tells you to press start. Right after the new game begins (when you get control of John Connor and hear a guy ask "John are you okay?") quit out back all the way to the main title screen again
Step 3 - With the Main Profile/first controller, select Replay Scenario. Choose "For The Resistance" on Hard and Co-Op, joining with the Backup Profile/second controller when the game tells you to press start. Now beat this level. If you don't have anyone to play the Backup Profile/second controller, just have it idle while you do all the killing. It's a very easy and short level, even on Hard. After you beat the game the Backup Profile will get the achievements "For The Resistance", "Seasoned Commander", and "Veteran Commander".
Step 4 - Now exit out to your Dashboard, go to My Xbox / System Settings / Memory / Hard Drive / Games. Scroll down to your Terminator Salvation save file. You will notice both of your accounts that played the game have a save file created now. Now, DELETE the save file for your MAIN PROFILE. If you own a memory card, back up the save first if you aren't sure about this or don't trust me. After you delete the main save, this will only leave the save for your Backup Profile
Step 5 - Boot the game back up again. With the Main Profile/first controller start a NEW GAME on Hard and Co-Op, joining with the Backup Profile/second contrller when the game tells you to press start. Again, right after the new game starts quit out just like in Step 2 and back out to the main title screen. I cannot stress how important this step is, if you don't do this you WILL NOT get credit for the Medium and Hard achievements
Step 6 - With the Backup Profile choose Replay Scenario and select "For The Resistance" level on Hard in Co-Op, joining with the Main Profile/first controller when the game tells you to press start. Again, beat this level. This time when you complete it your Main Profile will get the achievements "Veteran Commander" and "Seasoned Commander" and you will now have 1k in this game without having to play all the levels on Hard.
credits to YouStayClassy at x360a