I beat it last week. The last level is a bit of a grind.
Since I'm still having fun with it, I decided to go for the last medal in this game (no Steam achievements on this, only in-game medals). So I'm doing a run on Super Hard difficulty.
The diifficulty name is no lie. You're dead in two hits. Enemies respawn after leaving the edge of the screen. They shoot like crazy. And your weapon is so weak, it's basically a pea shooter.
You have to get good with timing your shots and reflecting the enemies bullets with the bionic arm. The bosses take a long time because they just soak up so much damage. And it's better to just swing past most enemies instead of engaging.
As before, the first level is really tough because you have no items. The second level (Area 04) with the infinite infantry boss is brutal. That long hallway with a bunch of enemies before the boss room takes some skill and luck.