Around how hard are the endgame bosses on each dificulty?

I'm talking about the Endless Tower DLC bosses, mostly, although we can also rope Osquio and base Simon in with them. I just beat the last of them, Divergent Simon (peak boss btw), today, but I did it on Story difficulty so I don't quite feel like the accomplishment counts. That's not what I'm here to debate, I'm content with not considering myself on the same level as someone who beat them on Expert.

But that's what got me thinking, what's the actual difference between the bosses on Story vs Expert? I think the parry timings are around 0.1 seconds easier (frankly, I don't really care about that, considering it's still pretty exact and I have some framerate issues that make it harder to see the real timing), but what about boss stats? The damage the main story bosses dealt was pitiful, but the DLC bosses are still one or two-shotting me with every attack. The menu doesn't talk about enemy HP changing, but I've heard mixed reports about that. Does it change at all? Overall I know my accomplishment was worth less, but how much less?