Inferno tactics.

The enemy will primarily target high-level units, not Imps.

No, that's not quite right. The enemy (I mean the neutrals here) will sometimes try to eliminate high-level units, and we need to try to replace those high-level units with weaker ones, thereby avoiding the loss of our high-level units, and replenish the inevitable losses of low-level units with demons.

A 4th-level monster is more useful than a 3rd-level monster.

Not always. Would you really use an Eye of the Storm on demons?
And Hellhounds can drain even more blood if used correctly. Demons are expendable and should be used as such, while Hellhounds are a raiding unit (very high speed, large numbers, attacks multiple stacks at once, moves relatively early, has a lot of hit points). They bite in one place, then immediately run to another place and bite someone else...

Ah, there's nothing to fear, the enemy will take care of them. A stack separated from its allies is doomed.

They will take care of them, but to their own detriment because of the fire shield. And the more diligently they take care of them, the more damage they will inflict on themselves. This allows us to achieve a double attack from our side at the beginning of the next round, and as you know, the attack can be decisive: units with high speed (+speed-boosting artifacts), and Devils and Hellhounds don't retaliate against attacks (i.e., they can attack twice in a row without being punished). Of course, this is just a general scheme that needs to be implemented in reality during the battle... but that's the whole tactic: there's a general direction, and the specifics depend on the circumstances. You can also make mistakes.

All of them? Are you sure?

Well, not all of them, but a significant portion—the effect is still much stronger than even from a well-developed regeneration skill. In addition, the enemy may be wary of this resurrection and place their unit on the corpse, which can also be used. If the demons die in the right place, the enemy unit that stands on that spot will simply be removed from the battle.

This statement:
doesn't align with this:

I meant that where there is a choice between upgrading attack and defense, you should choose the defensive skill, learn Armor, and use artifacts that focus on defense (and speed).

Where is Mariussa?!

Well, she can be used too:)

Also, you can combine the attack of Ifrits with the use of a Wall of Fire very effectively. The Wall of Fire doesn't affect Ifrits, but those who attack them will regret it, especially if you have a hero with the "Wall of Fire" specialization (one of the elementals). Usually, players don't like fire magic, so the probability of mass protection against fire is relatively low (but it still exists!).