To reach level 150, you need to earn a total of 11'325'000 of ingame money.
That is more than 11 million!
You will have to spend many, many hours if you decide to get this achievement.
Any game mode will give you money, just play the one that you fell most comfortable with.
You can earn extra money (XP), by adding boosters in loadouts. They last for 30 minutes and can give you 25, 50, 100 or even 200% boost. You get them from battlepacks.
You can check this website http://battlefield.wikia.com/wiki/Battlefield_Hardline_onlin...
to see how much you need for each level and how much you need to earn between levels.
Best boosting method I can think of:
1. Get at least 4 players together
2. Rent a server, so you can kick ppl who interfere
3. Put match time to max, I think it was 1h 30min, plus max points
4. One person is on one team, 3 others on other
5. Meet somewhere, someone from 3 guy team puts down spawn beacon
6. Lone players keeps headshot killing other 3 as they spawn, earning kill, headshot and nemesis bonus. Nemesis bonus appears after 3 kills, I think.
And the trick is that nemesis bonus keeps increasing, and you can earn A LOT of money. I dont know if there is a cap for nemesis bonus increase, I myself got it to 410, but I was not boosting money.
I believe that with this method you can earn the achievement in 15 hours per person. If you pair it with double xp event and kill boosts, it can be done in 6 hours.
Or rent a server, play Heist and just keep going for cash, no killing, but it is MUCH slower!
NOTE: You do not have to own the Robbery DLC to unlock the achievement, confirmed by DJ KOOLAIDE.