Child of Eden

For it's stratospheric TA ratio, this achievement isn't actually all that hard if you've got the hang of the game's mechanics. By the time you're trying to get it, you've probably already played the level at least a couple of times, and that's a good thing.

The key is maintaining that x8 multiplier that comes from perfect octa-locks. That is, lock onto 8 enemies (your reticule will turn white), and then release the button *on the beat of the music*. Each time you do this, your multiplier will increase by one (and you'll get a PERFECT! caption), up to a maximum of x8. Note that this multiplier is only applied when you kill enemies *with another octa-lock*. Killing enemies with anything less than an octa-lock will score regular un-multiplied points.

Also note that your multiplier resets if you fail to release the button to the beat ONLY when you have an octa-lock. So feel free to spam the A button if you get overwhelmed occasionally, but try and keep it to a minimum, as you'll be scoring less for each kill that way.

So your goal should be to kill as much as possible with perfect octa-locks. You can miss a few, and reset your multiplier once or twice, and you'll probably still make enough points.

HINTS:

(*) TAP YOUR FOOT TO THE BEAT - seriously. Do it. Nod your head to the beat too. You need to be aware of where that beat is at all times to maintain the x8 multiplier. Don't be afraid to hold the lock-on for another bar or so to make sure you release it on the beat. Especially on Normal difficulty, you usually have time to make sure it's perfect. If you can, turn the volume up too - it'll help you 'get into the zone'.

(*) DO NOT USE EUPHORIA - if you finish the level without having used any of your euphoria (your smart bombs), you'll get a bonus of 100k points. This was what tipped me over the 800k boundary. It's much more challenging to get this achievement without that bonus. I'm not sure if you get this bonus if you don't collect all of the extra euphoria items, so make sure to grab them.

(*) LEARN THE ENEMIES - once you've played the level a few times, you should have an idea of how many hits each enemy takes, so you can plan how to lock onto them so that you consistently get octa-locks.

(*) OCTA-LOCK THE HELL OUT OF THE BOSSES - pretty much every shot you make against a boss should be an octa-lock. Especially on Normal difficulty, most of the bosses are not that challenging, and you have time to get octa-lock after octa-lock. Without doing this, you're unlikely to get enough points.

(*) DON'T PANIC - Even if there's missiles coming at you that require you be using your tracer instead of locking-on, make sure you release that octa-lock on the beat before switching to tracer fire, or release the button before you get 8 things locked.

(*) DON'T WORRY IF YOU MISS A FEW ENEMIES - unless you're going for the 100% purification achievements at the same time, or the Gold Star achievements, it's fine if a couple of enemies escape off screen. It's more important to maintain your multiplier.

Here's my guide for gold stars, which is 800k and 100%. This is the version with commentary, explaining what I'm doing and generally why I'm doing it. I've also uploaded versions without commentary, which should be linked on the video page.

The most important thing to remember is to keep your perfect multiplier going! The multiplier only counts for octa-locks, meaning you'll get minimal points for spamming. For gold stars, you'll want a combination of octalocks and spamming, but if you're not going for gold stars, it's best to always try octalocks, even if the enemy is about to get away. Your purification percentage takes the hit, but you get a few more points out of it.

If you can purify the music box and move onto the scrolling boxes with 400k and all three euphoria for the 100k bonus at the end, you'll have more than enough opportunity to clean up the remaining 300k.

Also, if you're having trouble with octalocks, turn down the sound effects in the main menu. I dropped it to about 20% and the beat was very clear for me.

This is are excellent guide for this level, for the Gold Star (100% + 800k) rank you definitely need some practice, but you can get the 800k in the few tries. Just follow the video, this is very detailed and useful guide.

Solid level, required a few tries.

Also, you should know how to make octa-locks (perfect 8x hit in the rhythm of music), without octalocks you do not succeed.
And, you should get and save 3 euphoria at the end of the level - this gives you 100k. Without this bonus get this achievement will be much harder.
Again, just follow the video and don't forget about gathering and saving euphoria and, of course, octalocks.

All credits goes to SajikoSakura