OlliOlli

You could do this achievement on Neon City as MAKA suggested, but I found a few levels before getting to Neon City to be much easier. One in particular that I've shown in the video is Base Pro 2. There is a challenge that requires you do to finish in 1 big combo, which gives you an easy opportunity to score some big points. The level itself isn't too difficult either if you can time the jumps right. My highest score on this level was over 4 mil so there's tons of room to just relax and do a minimal amount of spins and tricks and still have enough points for the achievement.

Good luck!

This is most easily done on Neon City Pro 5 (Career), which is unlocked completing all 5 challenges in the amateur version of the level.

Once you start this level, you're goal is to continue a single combo from the very beginning to the end, and then get a sick landing at the end. Don't do anything overly complicated, just stick to 1 grind that you are comfortable with, and then use spin in between grinds to build your score and combo multiplier. It is extremely important that most of your grinds are "Perfect" grinds in order to keep speed and also help build the combo multiplier more rapidly.

Please see the below video which shows how to get 2M in a Career level, then how to get 2M on a Spot:


Note: It can also be completed on Neon City 4 Pro, but I found it much more difficult.