The difficult thing in Nostura’s Fey Castle is dodging the dark magic spheres. There are three sets to start the course and one challenging one to end it. If you can’t make it though the runes however, you’re simply not worthy Just don’t step on them, ok? You can also use the speedup power before the two jumps. It’s not mandatory, but makes the jumps a little bit easier.
As frustrating as the spheres are, there are multiple obvious paths through them. I marked one of them for each group of spheres and hope it helps. You can take the last set of four spheres that move against each other one by one. Additionally there are safe spots between them. Use them, because trying to rush through in one motion normally leads to death. If you do not get hit on the course, you’ll earn the two achievements “Highly Skilled” and “Fey Marked”.
Inside the castle you need to fight through some trash. Endgame geared characters can probably just run straight to the end and kill everything there. But watch out, the mobs can hit hard. The Nostura fight itself is also pretty basic. Whenever she “grows weary” of the fight, you need to enter the yellow portal and dps down the five tears. When she wants you to “be still”, turn away to not get stunned. Additionally her Wither “beam” attack can be blocked by bringing the stone and box pillars between the two of you.