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By the way, resistance can easily be more than 100% (arithmetically). Thorgrim at level 50 will have not 20%, but 70% resistance, and artifacts will add another 30% - that's already 100%. And plus the dwarf's or unique unit's own resistance! So, a level 30 Thorgrim with artifacts and unique units can provide the army with 100% resistance. Well, you've brought up an interesting problem!
Added after 18 hours and 16 minutes
I decided to finish off the enemy in his own lair and fully understand the relationship between all resistances. The unicorn's aura gives 20%, regardless of their number; the horn and rear work the same - I checked. The next test: resistance expert with artifacts (20% + 30% = 50%), unicorn (20%), and dwarf (20%). With the sum of resistances (90%), the chance of hitting is 10%; with the product of chances, it's 32% (0.5 * 0.8 * 0.8). I tested it - in one hundred battles, it was 23%, in the other, 27% - the deviation from 32% is too large, even more from 10%, but very close to the option where (50% + 20%) = 70% and 20% - the chance is 0.3 * 0.8 = 0.24, i.e., 24%. I did another test - the same heroes, but on one side, only a combat dwarf (40%), and on the other, a tree guarded by a horse (20%). For the dwarf, with the sum (50% + 40%), the chance is 10%; with the product (0.5 * 0.6) = 30%; for the tree, respectively, (50% + 20% = 70%) the chance is 30% and (0.5 * 0.8) = 40%. And here's the result - for the dwarf, it's 7% (should be 10), for the horse, it's 45% (should be 40). It turns out that the dwarf's innate resistance is added to the hero's skill (and artifacts) arithmetically, while the unicorn's aura resistance works with the hero's skill by multiplying the chances. That is, for a hero with a resistance skill (20%) and artifacts (30%), the unicorn (20%) will only add 10% (0.5 * 0.8 = 0.4).
Dirty_Player - thank you for giving me the push, because truth is born in argument.