I have 29/65 onyx medals with all matches played to onyx. I left my Xbox on for approximately 4 months just idling matches n felt no symptoms that my Xbox was going to break. My recommendation to prevent problems with Xbox would be to keep Xbox in well ventilated spot to keep nice cool airflow for system n download game to HD so Xbox doesn't have to work so hard to play game. Just FYI I also have the old school white Xbox. Had it for 6 years now with no problems. Good luck to all going for this achievement.
IMPOSSIBLE is just another EXCUSE to not try!