Can I ask if you've accessed the harem scenes with your teammates (e.g. unlocked their threesome scenes at the bath and whatnot)? I'm able to do a New Game + on my own save, but I chose a girl. I'm wondering if maybe choosing the "solo" route WITHOUT accessing your harem doesn't give you the extra ending, and if it's intended by the devs for story purposes.
Other than that, use RPGMaker to edit the Boss stats (if you know what you're doing, that is, since you can really mess things up in there). Or ... well, since it's an RPG, grind your way to victory. The final boss is INTENDED to be extremely difficult - I was able to BARELY defeat him after like, 10-20 minutes of back-and-forth. Other forum users (e.g. ULMF) complain about how impossible he is, but he's not invulnerable. Again, remember that he's the final, final boss of an extremely long RPG and he guards the gallery, which is basically a game-ending final reward, so don't expect it to be a cakewalk.