I just wanted to reply to this to say that this was by far the best help I've had on an acheivement from an online forum.
I played thru on Hard using this method, never replaying any of the missions or even trying to get high scores. No trying for headshots or anything either, just trying to get kills on the multiplier when possible. I stopped near the halfway mark on time on each mission, which was normally about the time I would accidentally shoot at a cop and have to run to the end to save the score.
I ended up at the top 20 on a few missions (10 on one mission), and am around top 20 in the world on score, and I havent even finished the final Big Boss mission. The Millionaire achievement unlocked around 60% of the way thru the game with this method for me, right on the money.
Considering I have a dozen missions I could add 100k per mission on, this achievement is ridiculously easy to obtain using this method. The only trick is learning where the gas tanks are, and knowing how to lure the cops near cars to kill them to get them off your back.
But either way, this acheivement shouldnt be hard anyways. I had 450k on Easy with just speeding thru the game, trying to get it finished to get the finishing achievement.