It seems Zog has the opposing red/blue spheres like the second of the zarlogs in the santuary. If you have blue and he has blue, his number increases and you can't damage him. But, if you have red and he has blue, his number decreases and when he is without color or only 1 or 2 of the oposite color as you, you can damage him.
Khisis seems impossible, couldn't get him to where I could do any more than 10-15 damage per hit.
Zar seems equally impossible.
I am guessing you must do these in a group, but I'm not totally sure since it doesn't say "gather a group"