Piece of Cake
To obtain this achievement you must finish the level Dawn on any difficulty under 15 minutes. I recommend you do par scores and Legendary speed runs first if you plan to, but if you want to do this one right away or if you already did the other 2, go ahead and do the mission on easy running past enemies. You can use the video below as a guide, but when your unable to do a glitch or de-spawn enemies, I recommend going the normal route and running past the enemies because you can spend more time doing a glitch then it is needed for the par time.
Note: I got this in in Legendary with 3 others, its quite easy, if you feel uncomfortable with trying just do the video on easy and you will get it your first try.
Legendary speed run:
Note: If you need to finish this one fast do it in Co-op, MUCH easier. Do 4 people, legendary and you will get around 10-15 minutes. You will still have to do solo, but at least you will have a much faster time for your "Your Joking" achievement.
If you are looking to finish Par time on all the Halo 4 levels, and also get the achievement for finishing Halo 4 in under 3 hours (You're joking achievement) then the perfect guy to show you how to do it is the person who gave the achievement name "You're Joking"
343 recognized ProAceJoker for the world record of Halo 4 legendary run-through by naming the achievement in honor of his accomplishment. If you absolutely hate glitches or roundabouts then you shouldn't watch him. All records for beating halo levels include glitches. Currently ProAceJoker is #1 on all speed runs legendary for Halo 4, he completed the Halo campaign in 1 hour and 27 minutes time total (half the time required for the Joking achievement. All under par time on legendary.
The video posted below is his tutorial speed run through the Halo 4 campaign he even stops to show you things and explains everything to help you out. If you follow his instructions you will obtain this achievement and more.
Credit for video goes to ProAceJoker