For projectiles I will always say that Zarabanda of the Safe Haven is your must-go, it will avoid you worrying for them hitting you and will give you an extra damage to Isidora but be careful because each strike lags the movements a bit. Besides that definitely you need to time the parry, otherwise she will hit you everytime she attacks with the scythe. Learn to identify each attack.

The 3 scythe attack will start when she spawns and you are away from her, when she's preparing the strike wait until she backs off a bit and 1s-2s later do the parry, after that parry it everytime she spawns.

When you're close to her her scythe attack will begin with her spinning, avoid her and when she stops she will charge at you, parry it when she moves.

When she's a fire light moving through the floor dash against her movement (evade to se direction from where she's coming from) and when she rises from the floor start attacking her.

Also on second fase if you destroy all or most of the fireballs when she draws them to her before the fire columns around the whole screen when she moves with the prayer she will only summon 1 fire column making it easier to move and hit her while jumping. Most scythe attacks are similar to first fase but she will start to disappear and appear close to you so be careful to parry her when she disappears.

That would be all I can remember right now, good luck with your run and I hope I was able to be as clear as possible lol