Did you upgrade your sword before leaving the village? Did you buy all the equipment from the shop in the village? Also, having a couple of potions helps just in case you get RNG unlucky.
I assume you didn't hence why you can't beat her.
You can upgrade your sword 8-9 times at the blacksmith before leaving the village.
One day you give him your sword, and the next day he gives you upgraded version v1.
Then next day you give him your sword again, and the next day he gives you an upgraded sword v2.
Hence and repeat like 9 times.
For the last upgrade, you will have to get the metal crystal from the cave north of the temple.
EDIT:
You must be registered to see the links
Download, copy it inside your data folder, and overwrite.
Start the game, go to Sarah's room, press on the bunny, and select cheat.
This will give you the best armor and weapon in the game since you can't manage on your own.