Battles with neutral creatures.

It's actually quite simple...
Either level up a combat unit and clear everything with it, supported by a mage, or create a strike force from weaker units, protect it and your heroes in battles, and sacrifice the rest of the troops and support units if necessary.

Exactly. A well-chosen strike force, supported by 4-5 heroes, can clear the map without any problems.

For example, I'm currently playing the Order campaign from the Whirlwinds expansion.
Which units should I use as a strike force at the beginning, when the hero is weak and the fourth level in the castle is blocked? I use Hobbits, protected by Golems and Dwarves:D They accumulate quickly, and clearing out neutral creatures, especially level 4 ones, is surprisingly powerful with Accuracy and Blessing). Yes, they are slow on the map, but there's no other choice; it's better to be slow but steady than not at all. Even when I reach Djinns/Nagas, I still hesitate to part with a couple of hundred Hobbits).

In essence, that's almost what you have to do, but I try not to change the unit type (because of morale and money). But you got the point - without a combat unit, it won't work; tacticians and mages can't solve the problem in such situations.

Here's another situation - you can't buy combat units anywhere for the Order faction until you find an external tavern or conquer another city.

Of course, if the AI tells a scout that he will be defeated, it means that with skill, you can, on the contrary, crush the enemy, and even with small losses - you assess the enemy's creatures and heroes, choose a tactic, and crush them. But if there is no answer to a problem, no trick will help until you find your own solution.
I once read on another site about tasks, about defeating much stronger armies, even without losses, I tried to solve one of them several times and decided not to solve them anymore.

And I, on the contrary, love to attack neutral creatures, regardless of the scout's warnings that I will be defeated). After all, in a non-online game, there is always a save-load option.

Sometimes, I have to replay a battle 10-20 times to defeat an army that is an order of magnitude stronger, and even without losing particularly valuable heroes or units. Although, more often than not, it takes only 1-2 attempts; 10 attempts would mean that the forces are simply unrealistically unequal.

Just recently, while playing the Barbarians campaign, in one of the missions, your hero is tracked down by a fierce squad of Grand Mages with several spells (Priest, Druid, and Order) tactics, a tank, a couple of dozen catapults, and several dozen crossbowmen. So, I ran into a situation where they caught me in a city - either I run away and give up the city, and they release a bunch of previously killed strong heroes from the city's prison, or I somehow come up with a way to kill them. And I only have a level 20 tank, an underdeveloped Priest, and a runner. And the army is just a bunch of weak units. It seemed impossible - you can't even survive on bottles - the Order Grand Mage will take and steal everything he can. So, I loaded the game 20 times to solve this puzzle:D

Such difficult battles greatly improve your skill in playing Heroes of Might and Magic 4, so even if it seems that there is no answer to a problem, think twice before attacking a strong army - you might find a way to defeat them.