Tips to Defeat Isidora Boss
Isidora’s fireballs do not count as normal projectiles so they can’t be destroyed by the “Heart of the Single Tone”. You can use “Zarabanda of the Safe Haven” to destroy a lot of the fireballs, but it is not a perfect defense. Despite Zarabanda’s imperfection it is probably one of the most efficient Prayers for this battle. Efficient only in terms of Fervor cost to active time I mean.
The Prayer “Tiento to Your Thorned Hairs” can protect your from everything as long as it is active.
“Saeta Dolorosa” will help you to regain hit points without relying on Flasks as much.
If you equip Rosary Beads that increase Fervor Generation, like the “Crown of Gnawed Iron” or “Embers of a Broken Star”, as well as the Mea Culpa Heart “Heart of Cerulean Incense” you can spam your chosen Prayer a lot more often.
By using a defensive Prayer you can equip more offensive equipment, like the “Heart of Oils”, “Knot of Hair”, or “Token of Appreciation”, and just try to end the fight before your Fervor and Flasks run out.
The “Cloistered Ruby” is an exceptionally powerful bead that can tear bosses like Isidora to shreds in a short amount of time.
There are a lot of good options for this fight, if you don’t have those items, try to do something creative with what you do have, or come back to this later. You should have the maximum number of Rosary Knots and Bile Flasks before you face Isidora, just because she unlocks mid-game does not mean she is a mid-game boss. Isidora is an optional boss and is balanced to be tougher than the end-game bosses.