Tactics for the Swamp (Fortress).

The same Tazr needs to reach level 20 to get a more or less noticeable 30% damage reduction, and I don't even take 15% into account in battle because the damage range is much larger.

It’s a serious mistake to think that 15% is insignificant. Even 15% makes armor one of the most important skills in the game. The point is that these 15% save units during the game: somewhere they will kill off opponents for longer, somewhere someone with several hit points will survive, etc. And I won't even mention Dragon Utopia and Ulki, where many enemies attack us at once.
By the way, armor also works well in combination with the Shield spell, and the difference (Attack (enemy) minus Defense (ours)) plays a significant role.

Hydra kills 3 in a stack, and each stack deals 35 damage, x5 = hello Hydra?

I hope it doesn't need to be explained that the hydra's enemy cannot retaliate, and gnomes with speed 3 still have to catch it.