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Iv'e had halo 3 for a while but just now started to get the secrets. In the vidmaster challenge 7 on the 7th do you just have to play the 7 on the 7th playlist one time every day from the first of the month.
Win 7 times or more on that playlist untill you have 7XP on that playlist. I really wish they would change the playlist though; it's getting annoying.