The full description on the Comeback Kid achievement says it has to be in xbox live play, it doesn't work locally. However, it doesn't say ranked play, so try it with a friend in a player match.
For the million points, we did it with 4 people on Give it Away. Vocals and drums on Medium, bass and guitar on hard. Final score was 1.49 million, so there's plenty of leeway to do it with three people or knock down the difficulty some. The bass part is pretty easy even on hard, with only a couple "tricky" parts. Vocals are mostly all talky parts, which work brilliantly right now with the vocal detection.
The million points can be done pretty easy on several songs if you've got 3-4 decent players, the key is to keep the multiplier going. It definately doesn't have to be on hard or expert.