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I didn't change the skeletons' stats; I couldn't come up with anything simple and understandable. I modified the AdvMapMonsterShared files for the alternative grade – instead of it, the standard grade will appear on the map.
I had an idea to give everyone protection against arrows, but that's just silly. I can imagine a skeleton resisting arrows, but lightning from titans and explosive shots should still damage them. So, it would be better to give archers resistance to arrows, creating something like a rock-paper-scissors principle: archers are better against infantry, infantry is better against archers due to the difference in attack/defense, but first, you need to break through the enemy's defenses. This means focusing on balancing armies rather than the physical characteristics of specific units. However, such radical changes would require reworking all the units.