Wow, Grandmaster boss is nuts. I had a pretty solid build I think, but I couldn't get him past 600 health. The only issues I see with my build is I had to fill up 6 inventory slots with keys to enable the key blade's damage, I went in with no consumables when my build relied on philosopher's stone to put out crazy damage turn 1, and my shield only had 12 block. That said, I was able to put out around 100-150 damage a turn if I didn't have to dedicate any energy to block. What really screwed me was the massive amount of burn he built up stopped me from being able to use the vampiric halberd to its fullest, thus nullifying my ability to heal throughout the fight.
I feel like if I dedicated any more space to armor, my build would be severely compromised and would limit my ability to put out massive damage turn 1 through philosopher's stone, and the payout for having armor instead would have been pretty small throughout the rest of my run, although it definitely would have helped with the burn.
I don't think the Grandmaster is unbalanced per say, but I don't see how a damage-focused run could ever beat him seeing how much damage he puts out every turn. The reason I built the way I did was to blast the curse and poison enemies turn 1, and I made it through basically the entire run without much issue, never really falling under 20 health. I think the key to beating the grandmaster is just to build something that can put up 20 block or more a turn without having to spend energy, and to come in to the fight with at least one of the potions that cleanses statuses, although 2-3 would be even better. From there you would just need reliable damage output of 40-50 a turn and you could eventually chip him down, although I am not sure if you would be able to get through his summons at that rate. 