First, you have to load up the mission that you want to beat on Veteran.

Then, play through on Veteran until you get to the first checkpoint.

Once you've done that, save and quit.

Load up the same mission, only on Recruit.

You then have to exit to the dashboard when the loading bar (the white bar at the bottom) is at about 33%.

This can be done by pressing the guide button then exiting or ejecting the disk.

Go back to the game and press resume game. Your game should be easier, but not by too much.

This glitch gives the game recruit AI, but you still have Veteran health.

Also, they still throw a large amount of grenades.