Canyon bosses are literally unkillable

Here is a picture relating to the problem "https://imgur.com/a/U6WML"

I thought at first that the poison cloud was somehow making them undying, but I never managed to get them down, neither by bleed, ignite, physical, fire, cold, lightning and auto-attacks. As you can see in the picture as well, they are both standing at no hp at all, but never managed to kill them.