I am not sure I really liked this episode.
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Jake's stupidity seemed rather exaggerated. Sure, he has never been the sharpest brick in the tree, but he has always cared very, very deeply about Finn. His initial casualness of Finn being i great danger seemed Out of Character for him.
Also, aside from the Lich being back and Billy being dead, things basically snapped back to normal at the end through a practically literal Deus ex machina.