Invisibility for the Town Hall.

But what if the witch, who isn't affected by the mass slow, carries the whole game? What kind of army is so easily defeated by a mass slow?

Who knows, maybe she will. Anything can happen in Heroes. Or should we just ignore mass spells now? They don't matter, according to your post. What's -40% initiative?

If it's a couple of dozen, then either I'll bring the whole army onto the field, or I won't engage in such a battle at all if it's not worth it. But if it is... then it is.

Actually, I thought you were going to lose "a couple" of units by deploying the entire army. No, against strong archers, invisibility and more invisibility are the key. Why deploy the rest? So they can take damage?

Well, it's hard to say. Too many "ifs." If the cavalry faces fast units, if you're playing against Inferno, if..., if...
Yes, cavalry is always targeted. But if you have regular, ranged witches, in some cases they can become the enemy's priority target, not the cavalry. Everything is relative, but the point is that the alternative upgrade for witches – the "shamars" – doesn't win you the game.

There are only two "ifs." And the second one happens quite often. I don't remember witches ever being a priority target, even for the AI, let alone a human player... I'm just saying that witches with their invisibility are no worse than ranged units. I don't know what to choose sometimes, but when I take melee units, I've never been disappointed.

3 turns in the final battle? In my opinion, the average final battle lasts about three turns if the opponents are equal (and if not, then no witches will save you). And then there will be nowhere to go.

Witches sitting in invisibility can be considered support units for your troops, and then, when the invisibility ends, they will be our reserve. And there's a big question of whether they will help or not. After all, level 6 with decent damage.

And even if you're fighting with the whole army, the risk of defeat is still lower.

Yes, attacking titans with only invisible units carries a certain risk, but at most, it's one or two shots at our troops. That's not a defeat. And without risk, you can't win in Heroes.