Had that problem in the past so every playthrough now I make certain to buy an extra crossbow from the blacksmith.Is it possible to glitch the game out with the crossbow? I have both the bolts and the shards but the Master still goes super vamp and wrecks me.
It means you end up with two crossbows in your inventory but you can always sell them afterwards.
I fight the master by alternating crossbow and wither spell, he rarely lasts more than a couple of blows that way.
Every time he scores a blow on Oksana or the MC use healing potion, you may need a lot of healing potions but eventually he misses and you can get a strike in.
Save before entering then if you lose you can replay.
Also equip the two purifiers with daggers and remove all the rest of their equipment before facing the master, you can sell that gear to cover the cost of the crossbow.