Select the map Junkyard. You're going to want to do this with 2 people.
Basically, you and your friend will stand on opposite sides of the Man Crush. One guy will leash pull (or kick or slide) an enemy, and the other will leash pull him through the Man Crush. This will earn you bonus points for working together.
After the first round, upgrade your leash at least once. I usually would upgrade my leash once, PMC power once, and defence once. After the second round, upgrade your leash again. This will make it recharge much faster.
Continue on this way, upgrading your personal stats, leash, PMC, and thumper. After about 5 or 6 rounds, you're both going to buy the Flail Gun. When you fire a flail at an enemy, they will get wrapped up and eventually explode. Basically, you'll flail an enemy, and before he can explode your teammate will leash pull him through the Man Crush. This will net each player an additional 100 point Sadistic bonus(trust me, every little bit helps). The flail gun is also very effective against the snipers who dodge your leash and kicks.
You are also going to want to complete as many team challenges as possible, as each person gets 500 points for a successfully completed team challenge. Enemies will glow blue when a team challenge can be performed on them. If you don't know how to do the specific challenge, you can hit the back button and an explanation will pop up. I found it helpful to clear out other enemies before attempting challenges.
Bosses are another good opportunity for points. If you and your teammate both kill a boss by shooting it then you will get bonus points for that as well and the bonus points for a boss team kill. You can also make things much easier on yourself by using your thumper on bosses. It comes in handy later on when the bosses are surrounded by groups of smaller enemies. If you do use the thumper and small enemies go flying, your teammate can shoot them out of the air for bonus Trap Shooting points.
Even with all these points, this achievement will still take quite a bit of time and it feels like a grind after a while. But this is the fastest method I have found by a long shot. You'll need a total of 1.9 million sp to reach max level. Good luck!