Is it any possible ways to get a costume? Is there any requirements for that? Does it happens in special time?

I have tried this about 20 times, and I get worked over badly every time. My guys are level 7. I have no idea how to beat this guy. Once the swarm of little bats start appearing, it's all over for my guys. What is the basic strategy here? I have all legendary weapons and armor, but I just have no chance against this boss. Very frustrating. Any help is much appreciated! Thanks!...

From the changelog, the event should run until jan 2nd, but I can't see it in game. My party is level 20 and I'm near the end (Earth Temple), but still no event. Did it only appear on the first year? Is there no way to get costumes anymore? Thanks!

How do I get the heart blossom?

I'm not very good at games :'( Is there a difficulty setting in the game? if not, how hard is the game?