Mine is 1.5 million points. I achieved it 2 days ago, Lost Platoon on normal.
I mostly drove around in a Chopper (and who doesn’t want to? It’s the most overpowered vehicle EVER, and a lot of fun to use) splattering everything in sight.
Took me around 136 minutes to complete. I got on a roll with the skull multipliers, and after I hit 200k it got easier to rack up the points. The wraiths added a nice bonus of 11,000 points, and with the laser it was a breeze. I got the hang of ramming the enemy Choppers, which gave me 3,000 points each. All in all, it was extremely fun, but got boring after a while.
Finally, life caught up to me, so I had to stop at 1.5 million. But it was fun.