I have been avoiding this cheevo like the plague! I only had about 14 or so gold missions completed after beating the campaign. Just found out like 2 days ago that you only had to complete the missed challenges instead of having to get them all in one shot.
So today I sat down and just knocked out a crap ton! I finished the day with about 45 or so gold medals. Pretty good start I think. Hopefully I will get 70 in the next few days