I'm playing this game for month and still can’t understand how can I defeat Quetzal Goddess. Does anyone know how to do it?

Deal a lot of damage while she's in her starting waiting phase.

I just always spam attack her until she dies I don't think ive ever lost to her O.o

Getting the Gladius from the infested ruins does wonders for beating her. And since you have the grace period of her not attacking, losing your balance isn't a huge deal.
Spring Attack > Tempo Attack until falling > Hop up > Spring Attack > Tempo Attack is pretty reliable with the Gladius and 6-ish STR / DEX / END, especially if the skills are trained higher.