Chakan the Forever Man, that game is fucking murder.
You beat
those levels? Here, have some more. By the way,
these are even harder. Hope you like instant death. Wait...no, seriously. You beat
those levels too? You need to get a new hobby. Here have an ending, and have some credits while you're at it.
But wait, yeah, you thought you were done? No. Here have another boss. And one try. You sir, are welcome for your game over.
Repeat until insane, or until you defeat the thing - whichever comes first.
I managed to completely beat it once on the Genesis, but if I ever get the self destructive impulse to play through it again, I'd use an emulator and abuse the hell out of save states.
The game's difficulty did do a good job of conveying the hopelessness implied in the "normal" ending though.