Honestly, that boss fight is more like "unfun" than unfair (and maybe even poorly designed since the game doesn't prepare you really well for it), and you can't actually do a meaningful amount of damage in most of his attacks.
I made a guide on how to fight him.
By dashing through i meant actually dashing before he gets close, the dash isn't invulnerable (i think), but he's slow enough to not hit you in time.