To get level 99, you need to earn 1,000,000 points for the respective faction, to do this i suggest that you credit grind, shown by the video below (Credit goes to FUSION CHA0S) until you earn the ability
Superstar- this pretty much doubles your intake for points;
This earns you these achievements;
You also need to have completed the campaign once, go to my page-customisation-character selection, after you've got superstar, you can either play in offline single player (which is what i did) but will take a lot longer for it, or play online where you aren't the host- this will increase your points and is also doable in system link
Not split-screen, the higher difficulties are recommended as they earn you a lot more points but is achieveable via. easy, i recommend chapter 5-2 on any difficulty this will earn you a ton of points, i know this because a friend of mine grinded this achievement and got upwards of 50k per chapter which is 20 playthroughs of 5-2.
In a nutshell;
-Complete the campaign once
-Get the superstar ability
-If you can play online or in system link where you aren't the host
-Play chapter 5-2 as much as possible, this achievement is in reach of 20 playthroughs of 5-2
*Notes
I did this in 16 hours gametime in solo offline, i didn't grind 5-2 when i did i got 20k per run through on easy