I'm going to:
1. Clear out shop except for Fou and 2nd and 3rd MoonFes CE
2. Grind hearts
So that's 5 until unlock 30 -> 30 until the MoonFes CE -> 5 until shop clear -> 30 for the rest of the event
https://www.reddit.com/r/grandorder/comments/6tz565/users_guide_to_the_moon_goddess_event/dloo91p/
yep this is my strategy, but I also want fou cards. Since I already have 50+ rider and assassin pieces and nearly 20 monuments, I may leave them in store if I'm tired of farming.