First boss fight is impossible to kill if you don't know what to do with his three weak points.

Weak Points

To kill The Unmerged One you need to destroy the three yellow weak points on his head, chest and leg.

1. Start with the one on his head because it's the hardest to focus on.

2. Let him get close to a explosive barrel and burn him.

3. Now shoot his weak spot with charging shots.

4. After destroying all the weak points he dies.