No loss is almost always doable.
What are enemy controlling spells for ? Blind , sheep, fear, etc ... You disable his last troop and revive your army, then finish battle. End of story.

Not in that instance, because the battle ends right after. Guilford gets to cast the spell after he has already lost technically.

You can't prevent the damage from that armageddon, but I remember that he apparently was not very bright (it did pretty low damage, not sure how much exactly because I do not care about no-loss). So maybe even lvl4 creatures with high hp will not lose a unit? (and you can always cast stuff like peacefulness, ice prison, last hero, etc)

Umm, that'd not make the game more difficult. It'd make it more annoying for people who want to do no-loss games, yes... but for those who don't care, it'd be a blessing if the enemy hero wasted damage spells on low-leve cannon fodder instead of more useful spells.

I'm a biologist with far more education that you'll ever get (considering that in your country, "high-school" is considered a high achievment), and I am guessing you are a 15-year old who gets his kicks from insulting people.