If she stuns you, equip something that prevents stuns (like the ring you just got). You can still buff yourself with stat buff items and madness. you're probably having trouble because your difficulty is set to 9Ok...yeah....this is stupid....as much as i've enjoyed the base game this winterbell dlc is annoying. Every single enemy is better than god almighty.
I somehow managed to get past the stupid knight from BS1 and now Leiden......how do you beat her?! I can't even rely on my cocktail of guts/requiem/dodge. I can only smack her as she smacks me harder, and she also restores her health! And stuns me every 2nd turn! The only efficient weapon against her, is the jabberwocky scythe, because she can't be stunned!