Firstly, we need to consider the probability that we will actually find this subordinate unit. If a Dwarf has all the runes, the Necromancer will have to work hard, and remember that we have Eternal Light + a rune that removes all negative spells. Secondly, regarding skeleton archers: I hope you won't choose them when playing against Haven (their bows are awesome) or against mages, as they have 3 units that shoot. So, skeleton warriors are the best choice here; they have, like the knights, a 50% defense against arrows from allies. Your archers are effective if the enemy has few or no archers, or if you are still sure that they are better. Let's take the example of a Dwarf: Karl and Erling are both experts in archery, so how can they compete? In this case, 100% warriors are the best choice.