Since there is already an excellent guide above by ReDSHiFTD, there is not much else I can say in terms of specifics, so here's just some extra advice for this maddening challenge.
1. Take it slow and enjoy the ride.
Don't jump in with the expectation to master it instantly. Doing so will overwhelm you and burn you out. The game is essentially a work of art, so it should be enjoyed as such during your first run through.
2. Learn the rhythm mechanic.
This mechanic is described in more detail in the octa-lock guides. It is absolutely essential that you become proficient with this mechanic to increase your score. In addition to practicing the levels over and over again, you can look up the original Genki Rockets songs on Youtube to help yourself become more familiar with the beats.
3. Realize that the challenge is fair.
Many eastern 'quarter eater' games like shmups and on-rails shooters are heavily weighted against the player to the point of being unfair. This is not
one of those games. There is no ulterior motive to the difficulty here, only pure skill. Even the most difficult and chaotic sections of the game are designed in way that can be handled efficiently with the use of memorization, timing, and prioritization of targets.
4. Achieve an almost meditative state.
As a good friend of mine once described it, meditation is the full effort and unity of the mind and body when engaged in a specific task. This concept fits well with the gameplay and is evident in the very soul of the work. Focus on your involvement with game and less on the achievements themselves. I know it's a fuzzy concept to describe, but I implore you to just try it and see if it improves your experience.
5. Let the music move you.
This coincides with octa-locking and meditation, but it's important enough to warrant its own explanation. The music and lighting effects are almost always in sync with octa-lock moments and enemy attack patterns. Don't focus solely on octa-locking the targets, but instead focus on the beat. With proper attention, the music will act as a precursor to the obstacles and opportunities that would be quite unpredictable otherwise.
6. Train your eyes to notice the color purple.
This may seem obvious, but just a second's delay in player response could mean doom in the harder levels. The color purple is almost exclusively used to show enemy projectiles, so learn to differentiate it from the mass of sensory overload. This also applies in reverse with orange representing a target.
7. Learn when to step back and cool off.
Even with intense focus on the last step, failure is still inevitable. No matter how good you are, you will fail again, and again, and again. You'll get mad, you'll get sad, and you'll begin to lose proper judgement. This happened to all of us Eden veterans. Even if you've been playing the same level for hours straight, after a certain point it becomes better to attempt it another day. I'm not just saying this as a mental issue, there is evidence for the physiological side too. Our brains begin to reject and ignore repeat stimuli after overexposure, so I assure you that you're better off taking a break than continuing a marathon grind session.
8. Try out the different visual and auditory effects.
To help counteract the above mentioned sensory fatigue, the visual filters and sound variations could provide some much appreciated variety. Some targets even become easier to see under certain conditions. Be aware though that this can also work in reverse and make some projectiles nearly invisible, so use with caution. This could also help with grinding the Lumi's Garden achievement too.
9. Consider trying out the game Rez HD.
Child of Eden is essentially the spiritual successor to Rez, which was originally released on the Sega Dreamcast. Luckily the game received a HD remastering so it can be experienced via the XBLA for just $10. Choosing to play this game will provide extra experience with the gameplay type and reveal an interesting relationship between the two titles.
10. Don't underestimate yourself.
This is my final and most important advice. I personally had no intention of completing this game when I started. It was so tough that I instantly became discouraged from even trying. That was my biggest mistake. With full effort and genuine motivation, you can achieve almost anything in life. All pros start out as utter noobs, but the transition can only begin if you are willing to try. Good luck and may the reflexes be with you.Bonus tip:
For quick reference, you can print out the various guides on this site, and then store them together by using the Kinect as a fancy paperweight.