Hey, I went ahead and replaytested Trerzothar several times on both Normal and Casual. For normal, you can reach 100% but you can't miss a helpful rune (or Wither) and never click a harmful one.
For Casual mode, it was hard to get past 91%, so I tweaked the difficulty and uploaded a new version (2024-09-20). Same rules apply, just make sure you dont miss any good runes and dont click any bad ones. You may also need to stagger clicking certain runes to make sure your energy is high enough before you finish.