I'm not saying this particular boss is unbeatable. I've beaten DT on normal before. I'm currently at the 71st floor on normal, so clearly I've managed to beat two iterations of him so far (I have doubts about a third, but we'll see). There are however two separate issues at work here; one directly related to the boss itself and one only indirectly so that has more to do with general difficulty creep. It is the latter I've been talking about so far.
If we're talking about the boss itself, I'm not going to hide the fact that I think it's badly designed. I'm hesistant to call it "too difficult" but rather "too restrictive", although I guess the difference is pretty academical at the end of the day. I find the frustration is mostly rooted in how those restrictions turns the entire boss into an exercise in finding a team that breaks the rules the least, and how that makes the entire thing feel artificial on the same level as these dungeon tournaments when you're only allowed to use a certain faction. Something as simple as the evil eye mastery can invalidate a champion entirely. I would imagine it gets worse the smaller your champion pool is. I think the way the dreadbombs don't seem to scale at all is probably the major source of actual difficulty and is, to my knowledge at least, the only thing that doesn't. I don't really understand the logic behind that one.
The fae boss, in contrast, don't have these restrictions and I have no particular complaints about it. Nor do I have any particular commentary on how difficult it is since I've only had one encounter so far and beat it fairly easily, but the concept is interesting if not very auto-friendly with the AI being what it is. I'm not entirely sure how the scaling works for this one, but hopefully I'll get far enough to find out.
In regards to difficulty in general, I've already said most of what I have to say. But just to reiterate; I don't think floor 10 on DT normal should be considered a particularly lofty goal when there are 110 more floors to block progress on (230 if you count hard). I don't think there should be bosses that are significantly more difficult than others to beat for the supposedly same difficulty level. Despite what I've said so far, I'm not sure if that's actually the case for this particular boss or if it's mostly a matter of people not having adapted yet, but I have my problems with it that are unrelated to the success rate. I don't think bosses should rely on artificial restrictions that will block progress just for the sake of it (e.g. frost spider being literally unbeatable without HP burn and no way around it). Finally, if content is too easy, then add more on top of it instead of raising the bar on what is already there, because there will be players for whom the content is not too easy that you'll just end up alienating for no reason.