Walkthrough of HoMM3: SoD Campaigns

Is it really possible to complete mission 5 for Yoga and mission 12 for the Unholy Alliance on King difficulty? If so, how? Even the Duke difficulty was manageable.

Yes, I think I've completed all the missions at some point on the highest difficulty. I don't remember the details, but firstly, if the hero is a carry-over, you need to level him up correctly (as far as I remember, some scenarios didn't involve a carry-over hero), and secondly, you shouldn't drag out the game while the enemy has few troops.
There was a walkthrough on NG.ru. As far as I remember, you need to hurry for Yoga. And for the Alliance, they seemed to recommend gathering an Angel Alliance - and then going into the dungeon, finding the Power of the Dragon Father, approaching the castle - and using Armageddon twice, finishing off the rest in melee - it felt almost unfair how easy it was to finish...