Rip game using NXE. Browse your Xbox HDD and find that game, it will be under Content\000000000000000\titleideofcopiedgame\00004000\
First you want to rename the 00004000 to 00007000 (Using XexLoader just make a 00007000 folder and then move the contents of 00004000 in to it)
Then you want to copy just the small pirs container to your pc (Should be abut 40kb) do not copy the .data folder that isn't needed.
Open Le FLuffie then point it the pirs container you copied to your pc, when it asks for a data directory just point it to a temp dir or whatever.
In the drop down box select GameOnDemand, the tick DEV LIVE, then click fix package (Do not tick hash).
Now copy the file back to your xbox replacing the old one in the 00007000 folder.
Game will be in NXE under Xbox 360 Games, will have the achieve list, and won't ask for disc.