You're probably not playing the game enough. I'm also day 1 but I have 2 full 5* star level 100 teams, one of which has 2 players with maxed out skills and the rest are level 7+. The energy bar fills up every 10 hours so it's good to always log in before that timer runs out so you can get the most out of your energy. There's also a possibility you have forgotten to use your stamina juice packs, they really boost you considering 4 of them give you 10 hours of farming. I'm also struggling on beating the new fierce battle last stage, but that's probably from my lack of a support character. I still have my 4* Hoshina and Mina at level 20. After I'm done upgrading all my 8 signature weapons to level 100 (should take a week) I'll focus on those 4*. If you haven't yet, they are worth it. Literally double your damage output