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are there any requirements for the achievment (like it has to be legendary)and can u do it on co-op??
no just find them all if you think you have them all that may be your prob try a youtube tutorial.
i have a couple of times but the guy will never follow me down the hole in data hive
the data hive audio log will appear if you got all of the others before
It's not camping, it's holding the designated location.
Use this map http://achievementhunter.com/games/halo3odst/guide