Insanely Twisted Shadow Planet

You will easily get this achievement after you beat the 3rd boss. Every asteroid you beat adds 1 to the overall score multiplier (ie. you get a 1x multiplier for beating the 1st asteroid and a 2x multiplier for beating the 2nd asteroid). You probably won't even need to beat the 3rd boss to get it. I played a game where we died on the 3rd boss, but we still had enough points to get the achievement with the 2x multiplier.

I had over 100,000 points after the 1st asteroid, after beating the 2nd asteroid I had well over 300,000 points, and after the 3rd boss I had over 600,000. The achievement unlocks when you finish the level and get to the scoring screen.

There are already some great guides on this site for beating the bosses, so I will direct you to them for overall strategy. The only tip I will give is beat all the challenge rooms (rooms with red spikes) and use lasers. Two guys with lasers and one bomb carrier will let you rip through this DLC.

Special thanks to Tommy Gun CRG for help with these achievements.