There's a switch that activates when you don't have enough RPM for the shield, which rolls the "No Shield" message later. I didn't add a line to turn that switch off anywhere so you just keep getting the message forever ♡ thank you for the report (you're not taking full damage since that switch is just for the message)
There are 3 bosses which are oneshottable by design, the witch, the cyclops and the old man. The witch is because she has absurdly high evasion, the others are to show you the world is not just about killing whos in front of you like a machine. But if your build is strong, some other bosses also have a mediocre HP stat and can fall down to a good hit. If you can tell me which of them is dealing 600k I can tell you if it's a bug or a feature, because there are some which you're not supposed to let touch you, unless your charge output is good enough to afford all the reviving.
The attack stats are really just your DP x RPM. A few bosses mess with it, but the witch in particular isn't trying to kill you, just make you weak so somebody else can kill you, so I think it's likely it's her you're encountering and becoming that weak. Some encounters have special conditions to give huge debuffs to your DP or charge, but she's the one that does nothing but that naturally. If you'd like a list of all the bosses that debuff you I can tell you, so you can avoid them (assuming you got their names unlocked).
GRR is just for a turn, when your stats show up again on the next turn it has passed already. On your defense turn you'll see that the numbers between () for Third Eye and Flow have changed according to your Bonus parameters.
Also, thank you for the first spiral report! Personally, the spiral helped me more often than not in my games, so I'm always thankful for her. But you'd never guess it from the outside, but sometimes the spiral affecting the enemy actually is helping you, or at least not hindering you, as some buffs do nothing for certain enemies. Sometimes you will find yourself getting 100k on the face because of the spiral, but I assure you the odds of her helping you are supposed to be greater than the opposite!
Thank you for all the reports, seeing how you're trying to engage with the mechanics and finding out where there's friction is great intel!