Be advised this is for the Halo CE campaign only. The other campaigns werent fully tested and its possible those have skulls that will null this 'loophole'.
This is not as much of a solution as a general tip. If you complete the campaign on any difficulty first, you can get mission/campaign completion achievements on higher difficulties with some of the more helpful skulls activated (mainly Bandana, Boom, and Grunt Funeral) by going back to the mission tab on the main menu. They do not pop as you beat the mission like they do when you do not have those skulls active but after you finish any number of missions and go back to the missions tab, they will start going off at that point.
This works for all difficulty levels so its suggested that you first complete the campaign on easy then do legendary next. With those skulls on the legendary campaign (which in my opinion is the most frustrating of all four games) is a lot more tolerable.
[with thanks to evil terrorr and AndrEvil]