Easiest way to this is to go to the survival mission called Boneyard, located in the Grand Senora Desert. It's the one that's an airplane scrap yard. This is probably easier with a friend if you are a low rank or lack money for ammo and stuff. Once you start, to your right should be a metal container. You get behind one side and your friend(If you did this with one) on the right side. If you did not do this with a friend just take off enemies from both sides.
If you do this with a high enough rank you can put sticky bombs on the road you spawn behind for vehicles, and use RPG or Minigun for any helicopters. If you do not have sticky bombs or and RPG/Minigun, then you just pick them up when they spawn. You can look on your minimap and you will see the weapons' shapes when they appear. Once you get to wave 7 or 8, enemies wil have Miniguns. You can just kill them and take their Minigun(s). After every round, be sure you are full on health and armor. If not, then go pick up body armor or health packs scattered around the map. Just be sure to make it back in time before the wave starts or you may get gunned down. When you are low on health and the wave isn't over and there's enemies running around shooting everywhere, it's good to have snacks to increase your health. The snack that fills the most health is the EgoChaser, but it's still good to have 30 P's & Q's for when you run out of everything else, so go buy max everything except for smokes and Pisswasser because they do not fill health and smokes actually take down health. Try using weaker guns in the beginning because enemies get stronger each wave. If there is only one person left, play it safe and send one person at a time. With taking down vehicles, it's easier to just wait for them to park and get out. When a helicopter come, shoot the gunners and then shoot the pilot. Aim for the head, practice makes perfect. Avoid all damage at all cost because one hit can take away 1/3 of your health.
Each kill gives 10 RP.
Each helicopter kill gives 25 RP.
Completing 10 waves will give around 30,000 RP and $20,000. You can keep repeating this to level up faster.
Thank my friend Dropback Legend for showing me this!
Hope this guide helped. Congratulations on your new achievement!
If you still can't do it, msg me and I'll help. GT:BKD Vevil. I can only get online every other weekend so I may not be able to help at that time, but I will check my inbox every time I go. Now, you can finally get done reading all this boring stuff and get your achievement!