If you mean the catgirl at the end of the forest area .
The best gear to fight her are the short sword and the herbalist cloth . The short sword has the highest damage and the herbalist cloth offer the best defence and the herbal heal magic which heals 100hp. The herbalist cloth even offer a resistiance to bleed and poison, which good since the most of catgirls attacks inflict bleed.
Btw It would probably a good idea in general to collect all armours, because when have the armour equipped while encountering a girl you again SP, when your armours SP are full you learn new permanent abilities, you can use even when you don't have the armour equipped.