Force Field.
Complete the "Dragon Slayer" campaign in Armageddon's Blade.
And there, in the mission with the fairytale dragons, you'll understand everything when you encounter the Legion of Nagas, although, if you hurry, you might even make it to the Darkness...
And in general, you've already mentioned one of the ways to use the "Field."
Also, another option is if the enemy doesn't have archers, and you have one very powerful unit, let's say a Titan, then you can lead it into a corner and constantly cover it with the field, and the enemies won't be able to reach it))))
There are many ways to use the "field," and no one will tell you all of them, either because they don't know them or because they simply don't want someone else to know their military secrets and not know what to expect if they play against them, say, in the tournaments on this Forum))))
