This trophy, is probably one of the most hard to obtain (in hardcore playtrough), but if you have a good solution, this is so easy.And internet is the best friend of the gamers who want to finish their games, so i post some solutions that can help you.
This is my favorite solution, very easy.The location of the military airport is
X : 28412 , Y : 5704. (video : IIWastedII)
This solution is good too but, when i try to do that, the guards kill me before the 5th kill.(video : Matt Reiland)
This solution is the funniest way to do this.And not so hard.(video : Stazzer)
If you want, you can do it with variants.(a plane with a statue, etc)