If you are having troubles getting the 2 million by yourself you can do this achievement with two players.
The positive side with this is the fact that you get a double bonus from the boss when you kill it. Killing it will wield you about 1.2-1.3mil, the rest should come from chains.
Practice by yourself and try to get a good chain going. If you do the intro of the level right you'll end up with a 21 or 22 chain, which is a good start. The rest of the level is not that bad either if you learn where enemies pop up. Remember that you do more damage against enemies with the opposite polarity of your ship, which is needed to kill some waves before they disappear.
When you play coop, let the second player stay back until the boss, to make it easier for you to keep your chain.
I did this on easy in coop with my sister who never played this game before, we got it on the second try with 2.05mil and a max chain of 40. Very doable even for new players.
Good luck.