DISCUSSIONS on “Rules for online games in HoMM3 and formulas for calculating player rating.”

In my opinion, you could give the Chupacabra a -50% attack penalty and prevent it from casting spells on Expert difficulty, while giving the Black Sphere a +50% attack bonus and leaving the skill resistance penalty as is. When both spheres are equipped, they would negate each other's effects, essentially canceling them out.
The cloak should be reworked, as Vic suggested, into an equivalent of the blue hat for level 4.