Looking from JP wiki, it's a raid boss hunt, meaning you will SPAM the event maps a LOT. Your objective is to collect event items and trade it for 5* and dress/skill blooms.

http://xn--eckq7fg8cygsa1a1je.xn--wi...B9%B3%E7%A9%8F

Apparently E-8 is the best one because it can spam 12 Million HP raidboss for 15 minutes which gives 500 for discover and 500 for poking.