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I've played many rounds already, and defeated the final boss several times. It feels that the boss's HP is kinda too low(?). The boss doesn't even have many chances to attack when facing the player firepower.
I don't really want to spoil anything, but there are certain objects in the world that you can find, and whenever you find one of these, the boss' hit points will grow a certain percentage.

As far as I know I found all of these items, and the boss had a ridiculous amount of hit points.