MonsterEnvy
Legend
His damage is in line with a CR 24. The party in the Adventure also has to go through a ton of Spyder-Fiends before they reach him they won’t be at full resources.I think your assessment is off. That theoretical damage is just that, theoretical; it doesn't take into account the many features and magic items a high level party has for mitigating damage, improving AC, providing temp HP, or locking down a monster. The absolute pitiful damage he's doing with his Legendary Action and the fact that on average his attack only does around 30 damage IF it hits just isn't that impressive for a CR 24, Spyder-Fiends or no.
Level 13 party can drop Forcecage on the Spyder-Fiends OR Miska if they wanted to. That's just one out of dozens of spells that can invalidate most of the challenge in this encounter. It isn't tough.
In the theoretical level 13 fight Miska can teleport. So Forcecage would not work very well