Requirements for Big Boss ranking:
1. No Kills
2. No Continues
3. 1 or No Rations
4. No more 9 Alerts
5. No more than 3 hours
Yeah ... good luck to you, sir. There was an awesome video floating around the web right after the game came out that showed some guy beating the game perfectly in under an hour. If you do a search you can probably find it.
You can also check Game FAQS for a walkthrough.