Currently, the boss has between 8,000 and 9,000 HP, and I set this value so the fight would last around 3 minutes on average based on playtests (without attributes). However, this might not have been the best decision, and reducing the duration to approximately 2 minutes or less could be a better approach.
As for the shield, I realize now that it was a design mistake. Bullet hell games generally don't include "perfect dodge" mechanics because they don’t work well with constant projectile dodging. Adding the shield ended up unbalancing the game by allowing the player to achieve near-immortality too easily. My goal was to make the mechanic as fun and rewarding as possible, but now I see that I need to rethink my approach. The best options would be either removing the shield or reworking some abilities around it to fit it better. In this format, I think the game would benefit greatly from more melee attacks with well-animated sprites that would also improve the boss’s movement, and second phases. Thank you for the feedback.