I got the game guide yesterday, and while I'm still stuck on this same boss fight (Barthandeus) I can see the end in sight. I just have to keep an eye on my timing for paradigm switches, and I think I can do it. According to the guide, the main story is about half the actual game content, and then there's the other half of it after the main story ends. So yeah, he's got a fair bit of gameplay yet to go.
I'm limiting myself to five efforts at the bastard fight per day, because I know me, and I know my enthusiasm will wear right off if I keep banging my head against a wall too long.