So the new feature to be able to play with a specific ghost on the bestiary, how do i unlock that option?, i got waboomi unlocked but i don't know how i did, is it only specific ghosts you can play with or do i have to do something specific?

You'll need to have done all applicable interactions with that particular enemy to be able to play test fight against it.

The blue button, "seen", is for seeing the enemy once. (Note that as of current, this progress is only saved once you go through a door.)

The pink button, "fed", is for befriending the enemy once via the endosomatic heart upgrade that you can buy at one of the shops.

The green button... Is a secret interaction. Here's a hint: It has a large role in hardcore difficulty!

Thanks for the explanation!, it is much appreciated