I'm not sure if anyone would be interested but being I toiled through the game without much help (I got a bit) I could try to write out some common stuff to look for. From memory I recall.. (spoilers below if you care)
Level 1
If you don't kill the first enemies soon enough, they don't all spawn.
If you kill the blocks that spawn the little worm things, they won't spawn worm things and you lose em.
Level 2
very straightforward level, just make sure the whale never fires on you or else you lost a percentage, otherwise you should always know what you missed.
Level 3
There is only one spot that I think has missable enemies, in the first area there are I think 3 flowers that can spawn 8 enemies each, if you kill the flowers before they all spawn, you lose em.
Level 4
This one is a real bitch to do purification wise especially on hard. Your first real opportunity to miss something is when you fight the running men, if you don't kill them in time they run away losing percentage. Then you reach a room with satelites that constantly spawn, it's fairly simple but personally I missed one of them a LOT, I found as long as i never moved the view myself, I always got 100 percent. Lastly is the part before the boss, this is by far my hardest spot to 100 percent because whenever one of the tiles explodes into bullets, you're screwed. Try to have enough points so that you can just rapid fire on them, on hard you need to beat the part with 29 percent, on normal it's like 33 or something I think.
Level 5
Oddly it's pretty easy to do this level but there are a few tricks, I never noticed it until I kept losing, but on the first boss the goal is actually to light up all the rings, just you can win without doing so, not doing so means no purification points. The next part you can miss stuff is when you get to the section with the whale, but for some reason the 3 smaller whales seem impossible to not purify as long as you actually try to shoot them.
I don't think you can miss anything else in this level until the end part with the tree which annoyingly does count, I must have missed one little light in this spot like 3 times -_-
Level 6
Not purification related but I made this on another forum, so i'll paste it here (the above stuff i just made for gaf though)
Wave 1
You can't get hit here i'm pretty sure. I put my controller down and didn't take a single hit, lol.
Wave 2
Very easy, just kill the spinny guys quick as I assume they shoot you (I never let them do it if they can)
Wave 3
I don't remember much of this wave.
Wave 4
Same as above.
Wave 5
One of the harder waves, when you see the little black worm things come out with the laser gates, pay no mind to the worms and just focus on doing 360 motions to knock out the laser gates, there are some other parts that are tricky but they are obvious in how to win.
Wave 6
This is just an onslaught of those annoying walls that get taken out by destroying the lights, just kill the lights asap, I found using the tracer on the ones with only one light worked well for me.
Wave 7
Kind of a break wave I felt, not sure what to say about it, I think this is the first wave you have to deal with the expanding walls if i'm not mistaken, if so, just keep using the tracer to expand the walls in directions until it gets big enough to explode.
Wave 8
Kill everything as fast as possible so they don't shoot you, lol.
Wave 9
This wave starts off a lot more intimidating than it ends, basically to start, try to kill the little blackish cube things but focus mostly on the laser fences in the middle, then go back to clean up the bullets if they managed to release them, throughout the rest of the wave it's not too bad, just kill stuff before they shoot ya and when you get inside the thing, it's really easy.
Wave 10
This one throws just about everything at you, my main advice is to kill those black cubes asap, especially right before you enter the finalish area, if you kill them fast enough they don't fire, and they are the biggest issue for the first parts, once you get inside the disco looking area it's actually really easy until you get to this part with rotating lasers, the best way to survive this part is to run your tracer right in the middle left to right (or vice versa) as it continually wipes out the actual threatening lasers (it's trying to intimidate you basically).
If you manage to pass the laser part, you're pretty much home free, some enemies come up but i'm not even sure if you can die there, if you do, you should feel ashamed as they are relatively harmless.
Hope this can help anyone going for gold