Yesterday, YouTube user Witwix shared a great way to get a good amount of money and RP in a given hour, thanks to the Mixed up with Coke mission from Martin Madrazzo. Though it seemed easy, it was a bit involved, and required at least four people to do the job quickly. You'll still need to do it in order to quickly level up to Rank 75, but once there, you'll be able to take on the mission Rooftop Rumble from good old Martin again.
Rooftop Rumble is actually an extremely easy mission to complete, assuming you have the right equipment. All you'll need is a grenade launcher and two well placed shots and the mission is over. That's it. Watch the video here.
Witwix also comments on a frequently asked question, "How do I quickly get to Level 45 so I can farm the Cement Truck mission?" All you'll need to do is a one-on-one race on the map Criminal Records in any Super Car, for a single lap. Farming that should level you up really quick.