The easiest level is 1-1. There's 4 categories that give you points. Combo, Time, Mobility and Weapon Variety. To get to 1,000,000 your best chance is to get a perfect combo (750,000), use as many weapons as you can while not losing your combo. Mobility will probably be maxed while trying the perfect combo. I did 1,002,694 without dying.
It would be nice to point out you can't get this achievement on any extra level.