I don't know. I did the mage tower challenge for Holy priest at level 912 or so. I found it hard, I tried 31 Times and I was really happy to get it.
A few weeks ago, I did the disc challenge and I found it super easy... 2 tries at ilevel 942 or so (before antorus opened). I definetely merit the holy one more.
I can't talk about any other challenge, and tbh I do not care that much about it.
ALso, I got my +15 key achievmentlike a week ago, something that I would have acquired a lot earlier, if I were dedicated enough
Just take your time and try. You'll have almost a year of content drought to do the challenges