All credits goes to
TheMightyGingeHD who is the author of the video below.
If you want to boost this achievement alone and without VIP privileges, here's a trick giving you 1500xp every 3/4mn:
1- You will need to finish the
Aftermath campaign first.
2- Load
A Few Complaints mission on
Insane in
Aftermath campaign.
3- Finish the mission on
Insane.
4- Then sign-in another account using a 2nd controller (can be silver or local).
5- Load the last save of the
Aftermath campaign.
6- Skip the cut scenes as quickly as you can by mashing
button.
7- Just play the scene (no ennemies to kill; just shoot 3 hooks: 2 on your path, 1 on your 2nd controller's path, you must shoot the hooks otherwise you will have to start over)
8- Get you free experience.
9- Repeat steps from 6 to 9.
Experience given according to each difficulty:- Insane: 1500
- Harcore: 1200
- Normal: 750
This methode isn't giving that much experience but it is the best I could find using no VIP pass and experience multiplier.
Right below is a guide showing you how to do it.