I need help: how do I defeat this bastard?

It's my first playthrough and for the first time I'm having difficulty with a boss. I'm stuck in the second phase of this boss, as I can't do anything when he expands his domain.

I ended up discovering that there is a quest where you receive an item that interrupts the mastery of the second phase, but I ended up losing the quest after forgetting to explore.

Is there any way to defeat him in his final phase?