I love the idea of the temple on the leviathan's head: very Appendix N. The structure of the scenario is very clear and there are some nice wrinkles added by environmental hazards. I wonder whether you could involve the leviathan itself a bit more? E.g. creatures spawning from its skin, a big hand swipes at the griffons, it can be distracted by piercing its skin or setting it on fire (so there are multiple ways to defeat it).