Game is not hard, just requires patience. Never advance on enemies, and you can beat the game without ever being close to get hit.
The boss is always circle-strafeable: Just keep X pressed and do many circles around, and it won't be able to land a single heart of damage.
Hard mode is much harder, because enemies home in the player, but boss is not harder, it has the same pattern.
Overall, a pleasant game, although I'm sure some sort of "magnet" would be nice