I haven't tried again since my last post, but I will. Thanks for asking! That's terrible... I attempt for a few dozens of hours then stop for several months. I hate this boss, he's exhausting.
Just wanted to add my support. (I've tried to route 100% for this game every now and then; it's an interesting routing problem. I suspect that optimal play involves getting the upgrades first, and clearing the bosses+hearts later; the route has to be different because you need things like yellow energy for SkySand. It's probably also worth splitting FireCage into two segments in a 100%; this isn't worth it in any% because it's such a detour to go to the save point halfway through FireCage in an any%, but it isn't a detour at all in a 100% because there's a heart down there that you need to collect anyway.)
Also, I can sympathise with your issues with the final boss; on the easier difficulties, I can clear the game with 97% faster than I can get the last 3% (Toughness above NightWalk, JumpBox heart, RainbowDive hard score heart; I've got the NightWalk Toughness upgrade once, but not IIRC the other two). The final boss clearly gets much much more difficult from one difficulty to the next, so it must be an absolute nightmare on Insanity.
Also, I can sympathise with your issues with the final boss; on the easier difficulties, I can clear the game with 97% faster than I can get the last 3% (Toughness above NightWalk, JumpBox heart, RainbowDive hard score heart; I've got the NightWalk Toughness upgrade once, but not IIRC the other two). The final boss clearly gets much much more difficult from one difficulty to the next, so it must be an absolute nightmare on Insanity.
