Weaker magic option is only available in the expansions, it would be great if we could have the option in the main campaign too.
I still stand by the fact that the AI is good on the tactical level, and I feel it is quite competent with magic too (they will use curse of shadows on archers or clumped troops, they will reactively use defensive spells against your offensive abilities ...).
The big problem the AI has is that you can customize a strong, synergistic army with hand picked hero skills and artifacts, while the opponent use more or less random armies (or thematic armies, in case of quest battles), with often only 1-2 heroes with a "strange" assortment of skills. So, even if their army is potentially stronger (with higher level higher tier units), your deck is actually pretty much stacked in your favor, and a competent (for a computer) AI can only do so much to mitigate the advantage you have.
And if there is a spell that need to be toned down, it is Soul Mirror

If you use it at the second after starting the battle, the enemy will be completely shutdown (if you built your hero around mana and mana regeneration).