Need help in Quest "A Soul's Bane"

I completed everything until this part where I am having trouble. The part where there is fake monsters as well as fake doors. I was able to kill one randomly choosing it, then I chose my second monster and it was fake. I looked in a lot of guides and it said that It would automatically disappear after 8 hits. But I hit more than 30 times and its still not gone. I am using a whip to attack it and this is the third floor I am having trouble at.

Anyone know what I am doing wrong?