What you were expecting to happen, and what actually happened?
Somewhere during the fight, the mana of one of the opponents changed to what you can see on the screenshot.
Funny thing: “Behind” this creature there is a hound (the thing that reduces your attack, don’t know english name).
When I killed the creature shown on the screenshot the hound appeared but it could not be attacked.
The fight ended with that hound still alive.
What are the steps to make it happen again?
No idea. Happened only this one time (Android).
Maybe do the fight again… It whas the Kerberos fight if I remember right.
Do you have any screenshots or video you want to share with us so we can see the problem? Attach them to your post!