I would recommend you use a kinematic body for the monsters, because 2D monster characters usually have very unrealistic movement, and Kinematic Bodies allow more control.
To my understanding, Animation Players are for animating 3D objects, or things like the node's rotation, position, scale, or other properties the node may have. In your case, you will probably want an Animated Sprite, which is used for sprite animation. (flip book animation style)
I hope this helps, and please comment if anything was unclear. :)