Guitar Hero II

Best way I found to get this Achievement is Free Bird - Lead on Hard, Rhythm/Bass on Hard.

You can mess up a few times but try to keep a 4x Multiplier as much as possible. Never activate Star Power until you have a 4x Multi.

Go through the lyrics portion of the song until Fast Part (One long Blue Note) so you know where the Star Power phrases are. You want to have a full bar to set off as soon as Solo A starts for major pointage. After that, activate Star Power as soon as you have it or if you know a section that would be better suited, but never before you have a 4x Multi.

I ended with 50k over a million with quite a lot of mistakes (Friends Orange button was playing up).

You need to be quite good at Hard and it would be wise to have 5 starred the song in Single Player before you start.

Good luck and I hope this helps.

Scoring a million points on a song in co-op mode is insanely hard, you'll need to be extremely good at the game and you'll also need to know someone locally who is too.

It's only obtainable on 2 songs*, those songs being Jessica and Freebird, now it's up to you which song you choose to tackle this achievement on but before you do decide let me give you some information.

Jessica is a lot shorter than Freebird but it's margin for error is minimal, you'll really need to nail this song and you'll both need to be on expert.
Tto date the highest score achieved for it is 1.1 million.

Freebird, this song is incredibly long and it can be very boring to play over and over when trying to reach a million points, however, with the songs length comes a massive margin of error, the high score for this song to date is nearly 1.4 million with both players on expert, to make Freebird even more enticing, you can both play on hard and if you play well enough you'll still manage to score the required amount, if you don't have a friend local who is great but you are (or vice versa), one player can play on expert while the other can play medium, even with this set up you can still manage a million points.

Of course, if you're both great at the game and can handle it, play on expert to guarantee yourself the million points!

I've linked in a video of 2 guys playing on expert and hitting over a million points to give you an idea of when to use star power, try to follow this star power path and so long as you don't mess up loads you'll manage the achievement!

^{*referenced from the http://scorehero.com high scores, GH2 co-op mode.}

*^{Credit to Kov and KoT for the video}

All of these guides have been stating that you have to do good on Hard/Hard on Free Bird to get the achievement, but for the people that cannot find a partner that can play this song well on Hard, I have an easier solution.

I just got this achievement yesterday with my girlfriend and what we did was I played on Hard for rhythm and she played Medium for lead (this is important that the Hard player plays on rhythm and the Medium player plays on lead because the rhythm player will get the more trickier parts that will get you more score in the end.) You're going to want to shoot for about 98% notes hit for both players. We ended with 1,044,428 (because we had 1,000,000 before the end wankery so we stopped trying) with me hitting 97% and my girlfriend hitting 99%, missing only 2 notes.

Be sure to save your starpowers for the two choruses, because a good chunk of your points will come from those triple notes that the person playing on Hard will be playing. After using full starpower bars on the two choruses, we just used starpower as soon as we got it for the rest of the song.

Hopefully these tweaks that I offer will help struggling players achieve this strenuous achievement.

Just play Freebird Hard/Hard, and follow the starpath that was used in this video (the players attempting this should be capable of playing Expert at a pretty high level). Both players got around 95% notes hit, and there were multiple starphrases that were missed so there is plenty of room for error.

I'm with [ J Lo x360a ] 100 %.
We actually got it the first time this way an we got like 1.1 million and we missed three or more star powers.

However, If you feel confident about yourself, you could always try doing "Free Bird" one on expert and the other one on Medium and this will work as well. Preferably the expert player play on Bass since it actually got more but still easier notes :)