Well, i dunno if my method is the best one, but i can sure for you that you're gonna win the fight with due time.
Since you can't deal damage with her from your attacks, you need another source of damage, in this case: Items
Dung Pie can poinson her for 2% MaxHP damage over time.
Oil Urn inflicts the "Oiled" debuff (Increases Fire damage taken and reduces Speed by 20% for 3 to 4 turns.)
Fire bomb can used to do fire damage (it does consume your action however, so use Oil Urn first)
Nightmare Lantern does the same as the item mentioned above but it also applies "Burning"(5% MaxHP damage over time that lasts 2 turns).
Scalpel Can be used to deal 10 (yes, 10) fixed damage and also deals the "Bleeding" (3% MaxHP damage over time that lasts 2 to 4 turns) condition.
Last item in my list is the Thorwing Knife, a knife which does damage based in your speed stat (it's a minor damage, but the important part here, is that IT DOESN'T CONSUME YOUR ACTION TO USE IT, which means you can use 99 of them in a single turn, usefull to do the last bit of damge for killing Florence)
Hope this mini guide helps you, i won against her second form just using those items, my regen equip and using some actions to guard.
Quick Edit: Forgot to mention the "Shadowy Bog" skill. You can get it in the asylum (Deals 3%MAXHP as damage, it's unavoidable, removes dodge buff and reduces all paramenters by 1 stage, pretty good stuff)