For this mod you will need:
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USB

--TEXT TUT--
1.Start a game on Halo Reach
2.Save and Exit
3.Go to dashboard and move the gamesave to your USB.(Its usually your profile name)
4.Plug your USB into your Computer
5.Run and if necessary update liberty
6.Select 'Select save file on a remove device' and hit OK
7.Select your save device in the drop down menu and it should automatically select your Halo Reach Save. Click OK
8.It now should show you your save game, if its right, hit Ok. If not, close the program and start again.
9.Tick the 'Enable for player (and current vehicle) invincibility'. We dont want to mess with the other settings. Then hit Ok.
10.Click 'MAX' for all available settings.
11.Do what you want here if you know what your doing, If not just hit Ok.
12.Enable 'Max ammo to all loaded weapons' then click Ok.
13.It should automatically re-save to your device. Once done, remove your device and plug it back into your Xbox360.
14.Move the save back to your Hard Drive or primary save device then load up Halo Reach.
15.Resume your Game and Enjoy! BTW This helps alot for legendary!