If the topic doesn't exist, you can create it ;).
I'm also bothered by this question. It's not a bug; it's due to the "Loyalty" ability – each unit receives 10% of the damage directed at Michael. You can cast "petrification," but that will only save one unit. But how about four...?
And just some thoughts: Petrification from the Mage + from the Earth Elemental in the army + a summoned elemental – that's already three :)
P.S.: Question, if you complete the game with all the children, will they all be present in the final mission? Or just one, chosen by you?
That is, is it necessary to complete all campaigns before the final one?