HELP ME PLEASE

So this is embarrassing, but I just started playing through Monday and I’m at the battle in the amusement park where Yumi keeps calling up the demons and she’s in some sort of trance and you have to defeat everything. I am having SUCH struggles, I’ve tried maybe three different tactics to approach the battle and failed every time. I know that my team is pretty weak, but I thought they would still be good to handle her since I had no trouble with any of the previous battles. My protagonist is at level 12, and the rest are either level 6 or 8, can’t remember. I tried doing that free battle to level up, but it was so hard that I quit midway. Should I like…restart the game from the beginning and level up my characters more before reaching Monday and taking her on, or is there some sort of trick to this that I’m not getting? I’ve tried exploiting the demons’ weaknesses, but they heal and multiply so much that as soon as I kill one, another one appears. What should I do? How do I approach this?