Hello! First of all, I’m sorry if the game felt confusing to you. I’ll try to improve that in the future.
The game works like this:
The synchronization bar only fills when the boss is exhausted and becomes vulnerable, meaning when it stops moving.
At that moment, you should get close to the boss and attack as much as possible to fill the soul synchronization bar.
Once the bar is full, the boss automatically enters a furious attack state, where you must complete a QTE event and synchronize your soul with its soul, causing it to absorb its own damage and reducing both the player’s and the boss’s maximum health.