You're Joking Achievement in Halo: The Master Chief Collection

As one of the four Legendary speed running achievements, this achievement will prove to be one of the most difficult tasks within the entire collection. It's worth mentioning that you may want to grab two or three Co-Op partners when working on this achievement for two reasons. One, the extra firepower will help put down Forerunner and Covenant enemies a little quicker. Secondly, one or two players can usually act as a decent distraction while the remaining two players attempt to push on and run by as many enemies as possible in an attempt to grab the next checkpoint.

This essentially means that you may want to replay individual missions dozens of times in order to perfect your runs and learn new tactics in skipping various firefights. Unfortunately, a text-based guide would be too tedious in explaining every single mission within the game, so please refer to THIS LINK for an in-depth look at completing Halo 4's Legendary Campaign in under three hours. A huge thanks goes out to Naked Eli for providing the video content.

As confirmed by Sun100, this achievement does not need to be attempted in one continuous run. Rather, the game will track your best time on each mission completed on the Legendary difficulty setting and add them up for a grand total. When a new "Personal Record" is set for that mission, it will override the previous tracked time. This essentially means that you can tackle a mission multiple times in an attempt to perfect your speed run.

NOTE: By following the speed running tactics provided in the video above, you will almost certainly obtain the Par Time achievement for each of the Campaign's eight missions. This means that if you have yet to grab the Par Time achievements on the Easy or Normal difficulty setting, they can be stacked on the Legendary difficulty setting while working on this achievement.