it 1 shots you and has 2k HP
if u wish to cheat you can open monsters.ini file and edit it.
She can't kill you when you have shield. Just like in the previous game, there is no overflow from damage that breaks your shield, so even with a 1 hp shield you can tank anything.
The trick is staying at the far left tiles. When you're there she can only use the scythe attack, which hits all 3 back tiles but has a long windup and you can easily dodge it. A max shield can tank 2 or 3 of these before breaking. You can go into the fight with a maxed shield from a previous fight, also if you took any damage, the mushroom girls' attacks heal instead of doing damage.
If you're standing anywhere else she will use a bite attack that is super fast and deals 4k damage, instantly breaking your shield if you have any or killing you.
Equip the upgraded mechanical eye so you can see her stamina bar. After it fills and you're standing on the right spot she will telegraph her attack, so you move to safety and back to the back. You can squeeze an attack or two in-between her attacks. The thunder move will only damage her shield, it won't do anything once that breaks. The meteor spell that you can unlock in your magic book can damage her.
Using E to cast spells in this fight is NOT recommended. Pressing E attracts the mana/arcana orbs that spawn on the battlefield and it will most likely make some orbs fly to her. You don't want that, orbs restore stamina to enemies when they pick them up. Better to be safe and not press E at all, also pay attention if an orb spawns directly below her, that will make her double-tap once the orb grows enough to be absorbed.