Load up Mission 2 (fairly certain it's mission 2... after you crash land on Requiem)
Book it to the area with the Warthog. Then just hit the guide button and wait til it hits 12. Then jam on the gas. There's a few places that will trigger it I think. I got it once you hit the cave-like area.
If you're setting your system clock, set it to 11:53-ish or so. That should give you enough time to get prepped.
you can also manually set your clock of your Xbox to 11:45 or so, then start the mission. If you miss it, reset your clock, reset the mission. This might have been patched with one of the updates, I dont know for sure. If it works, then you can get the achievement any time you want.
A lot of people might consider this cheating as well, I dont have the achievement yet, but my friend told me that this works.
God... The achievement does say to get air in a Warthog at Midnight right? So, naturally you have until 12:43 AM. Why would you have any longer than 12:00 to 12:01? Also, the Warthog is like 5 minutes away from your starting position if you dont know where you're going. You really dont even have to get any air, just get all the wheels off the ground. You can crash and roll the Warthog and still get the achievement. Takes all of 5 seconds.