OLOT's solution does work but only if you have met certain conditions beforehand.
The conditions are: you have already unlocked PR 1 - 5 AND
You have the rank of Predator/Best Guy.
His method for boosting to Predator is successful and super easy. You may wonder how I know this as right now I don't have the achievement. So, to answer that question, I know it works because I helped a friend, Moose of Woe, get the achievement last night and it took about 5 minutes for him to get his rank up to Predator from Ultimate (I was the host and had a rank of Greenhorn).
A couple things I should note here are that you must play the game regularly to keep the rank of Predator. Supposedly, if you haven't played in the last 12 hours your ranking will get downgraded back to Ultimate. What this means is that you will want to make sure you have all necessary conditions met (i.e., you have unlocked PRs 1 through 5) before you attempt to get the Predator rank so that you can go for this achievement right away.
Also, (according to Moose of Woe) crashing into the water will not affect your rank, but crash landing on the tarmac &/or getting shot out of the sky will. So, if either of those"bad" things happen, dashboard it so that the game doesn't have a chance to save your file.
From what Moose of Woe said, the hardest part about all of this is finding the planes you have to shoot down for PRs 1-5. He added though that they seem to appear quicker if you have a higher rank. Here is a great guide from the xbox.com forum's that will provide you all you need to know about playing this game as it is meant to be played:
http://forums.xbox.com/6021443/ShowPost.aspx (LINK is DEAD unfortunately - kept as a reference...but this is also a good guide to the game:
http://faqs.ign.com/articles/717/717399p1.html). This game is not easy by any means and OLOT's method is about the only shortcut there is.
Finally, after you have you have met all of the conditions to get PR6, select Big Boss mode to shoot down the required Big Boss. If you have gotten this far, you don't really need my help anymore though:).