Can you change the difficulty?
The launch version has no adjustable difficulty setting. Difficulty options were removed before release because of time constraints. Adding them later was under consideration, with no announced release date.
What to try when a boss feels too hard
Explore for boosters, spend Protoplasm on your blasters and skills, then return to the fight. An Evolve Chamber also lets you refund purchased upgrades and try a different allocation.
Use target lock to stay on an enemy while moving or jumping. For the darkness section of Possessed Amari, use the scanner to find the boss.