My route is mostly based off this video
, with the only major change that I get to Phantom Castle with lv.2 shot. I then proceed to destroy parts similarly to the video but doing a few piercing shots here and there to weaken parts a bit. I make sure to get to the part where PC turns to the side and starts shooting blue cancellabe bullets, yellow lasers and drones with meter for at least one full burst-detonation (this destroys a few parts but leaves the ray shooting ones) and then get back to build a full meter again. You build it VERY fast at that spot.
As soon as I get it I do burst+detonation twice at point blank, and keep shooting at point blank to regain meter for another burst+detonation. If you do things right this is enough to destroy every part before PC can do anything. It then starts doing that spin where it throws parts away, but it has none, so you can pointblank it for free for a good while. After it is done it almost always does burst right away, so you get a counterburst chance. I detonate, then avoid the lasers and burst+detonate again. This leaves PC at critical health. I know that another counterburst will be a guaranteed kill after this.
Then what is left is to pray. As DKA_Darius rightly says whether PC shoots another burst is random. Sometimes it does another just after going up/down once or twice, sometimes it doesn't shoot it for a while and it starts doing that fucking annoying pattern where it comes down spinning from above or charges right ahead spamming lots of homings that either wreck my multiplier or kill me, depending on how I did before. In any case you get much more time bonus than leaving PC loop his first attack sequence.
Another really annoying bit of RNG happens at Hungry Gluttons. That pattern where the little helpers form a wall and shoot straight ahead while HG itself shoots aimed pink lasers gives a LOT of points if bursted (with the benefit of not having to actually damage the boss so you can still do as many counterbursts as usual), but it's completely random. Sometimes it happens thrice or more and that can mean an extra 10 million, sometimes it doesn't happen at all. Silly.
EDIT: Small improvement. Perfect Phantom Castle fight (all parts bursted -well, not SO perfect, a few destroyed without detonation- and 16x counterburst finish, 148 million at the end of F), shoddy everything else. Particularly ugly Hungry Gluttons fight, missed a few counterbursts.
Hagane - 230.186.600 - J - Assault - PC