There is no quick way to achieve this. Regardless of the method you use, it will take a significant amount of time. However, there is one method that requires absolutely no effort.
In order to reach level 100, you must accumulate a total of 1,584,350 RP. You can get RP for various activies to include heists, missions, and more.
You can also farm RP indefinitely if you have access to a second Xbox and account. For the no effort method, ensure each of the consoles you intend to use are set not to shut down after inactivity. Ensure that controllers for both are wired to the console so that the batteries in the controller do not die. Now, take the second or dummy account and buy an office in GTA online (this is done to ensure eligibility to become a CEO.) One of the offices available to buy is completely free. Once the office is purchased, use the interaction menu (hold the
button) and select Register as a Boss, then select SecuroServ CEO.
Now, have your character join that character in the multiplayer session. With the dummy account, use the interaction menu, select SecuroServ CEO and select hire associates. Following that you can choose any number of options to hire specific players. Choose to hire your main account as an associate.
Now, find an area that will keep you safe from other players and take both characters to that spot. I suggest an apartment or house owned by one of the characters. From there, find a couch on which you can seat both characters. It is important that they are seated next to one another or at least very close to one another. Once they're seated, take both wired controllers from each Xbox and rubber band the two joysticks. This will prevent you from being kicked from the GTA online sessions for activity.
Approximately every minute you will get 200 RP as a reward for maintaining close proximity to the CEO. If you leave this running and have all the parameters set as above, you will gain RP indefinitely with no action required on your part. However, this method will take a very long time. With around 12,000 RP per hour it will take you 5 and a half consecutive days (12,000 RP/hr into the 1,584,350 is 132 hours, or 5 days and 12 hours.)
I have done this method now for 2 days and have gone from level 39 to level 70 and counting.