Event Missions Mii Costume strategy

I really really want to get the day of the dead dress but I'm a bit lost on how to do that. Do I just keep grinding event mushrooms? I feel like my pikmin set are too powerless to yield good results. I don't know how much time should I dedicate to getting the Calavera Decor Pikmin by clearing event challenges - like the more calavera pikmin I unlock the better I can be with event mushrooms right?

Ahhhhhh AHHHHHHH please send help