No, there's no secret. The event is the definitely the easiest way to earn an ascension material, especially for the demon heart. Currently, there's no free quest for getting the demon heart, that comes out after the first segment of the main story ended.
There's a way to earn few by clearing the interlude, but that is only if you have the certain servant. (For example, clear the interlude for the Blackbeard drops forbidden page.) Currently, only the interlude for Orion and Tamamo drops demon heart for certain.
Only thing I could say is, wish you the best luck.
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