The final boss after hell's gate can be accessed by using all four of those organic sex 'machine's at hell's gate with that gallery 'key' bar in the bottom right being completely full. Oh and avoid the jellyfish if you can cause it'll 'carry' you straight to having to retry using all the 'machines'. But once you use them all you beat the gate and it opens to the final boss.
As for whether it's beatable or not, it most certainly is as I beat it a long time ago.
But not without cheats and the cheapest of spam considering nearly all of the final boss's attacks/combo's equate to insta-KO, but finding all of them was quite enjoyable, my only complaint is that there is one that is way TOO hard to get, and you couldn't get the GO animation unless you didn't have infinite hp.