So this fight is at the bottom of the Mantis Village and is kind of annoying to redo due to some platforming required to get back from the bench.
I highly recommend picking up two nail upgrades before this fight from the nailsmith west of the City of Tears as it makes the fight way easier. One is just geo but the other you can get by acquiring the Pale Ore from the Seer in Resting Grounds.
To get to her you need to enter the Crystal Caverns, go all the way to the right, do a somewhat annoying platforming section then do a dream sequence. You wake up in the Seer's room.
Note you "can" get into the Crystal Caverns by using Soul Master's ground pound, but you'll have to do the platforming in the dark or get the flying ability out of the caverns, you'll be stuck in there until you do one of these. Save yourself the headache; grind up the geo for the Lumenfly Lantern in the Dirtmouth shop, then open the Crystal Caverns gate.
You need 300 Essence for the Pale Ore required which you can't even start till you meet the Seer. Easiest way to get this is to do the tree in Resting Grounds, the ghost warrior fight in Resting Grounds, and the ghost warrior fight up on the bluffs west of Dirtmouth. Both of those fights are easy with generous corpse runs. You get on the bluffs by going to the room the game starts in, going through a hidden opening in the left wall, and mantis claw climbing. You also have access to the tree above Mantis Village and another ghost or two but those are what I did.
Once you have 300 essence talk to the Seer (you woke up in her room after the dream sequence) until she gives you the Pale Ore. Take it and 800 geo back to the nailsmith for the second upgrade.
The first nail range upgrade charm is helpful and the retribution charm can be helpful (though I didn't take the latter). You should also be able to get at least one more mask by this point.