Once you join clan Centurio, it's based on the number of "Clan points" you have, IIRC. You can see your current ranking on your save file or in the "Clan Primer" menu. (Note, it may also be based on the number of marks you've hunted.)
You get points from fighting just regular enemies, and it's not very difficult to get the required 1,000,000 for the Order of Ambrosia rank.
Check out gamefaqs (
http://www.gamefaqs.com) for more info.