Saints Row: Gat out of Hell

I recommend you save this for last.

1. Collect all the collectibles in the game. This'll give you a fair amount of flight distance.
2. Complete ALL challenges, EXCEPT for the Upgrade All Weapons one. Most likely, that'll be the last challenge you need anyways.
3. Do all the other achievements in the game except for this and the Complete All Challenges one.
4. Now that you have done everything you need that involves flying, you'll probably be around 80% done with this achievement, and still need money for the Upgrade All Weapons challenge. Go to the Hellblazing Easy Activity in Drudes Crossing at the top right of the map. It'll be a 15 goal activity that will give you ~2000 meters every time you do it, which will be around a minute. Since you need money, you might as well get money while going for the flying achievement.
5. Once you get this achievement, just equip the Greed SMG and Vacuum Stomp and farm enemies to also complete your last challenge since that is much faster than doing an activity.

Here's what I did: I got everything else done with the exception of completing the final mission and a few achievements and challenges. Yet, I still was at 77 percent for the achievement. So, I put the quest marker on the final mission in the middle of the map and flew under the floating island. Then I flew back and forth from one end of the map to the other. When the game made me turn around I flew the other way. It was a good 3,000 meters each way, which netted me 2 percent toward the achievement. When I finally made it to 99 percent I flew to the mission and then flew in circles until I got the achievement. You can even complete other achievements while working on this one.

The following achievements can be gained while working on this achievement: