Who has the archer stack right now?

Yes, I hadn't really thought about that either. Although, in my opinion, the question of "how to best eliminate neutrals" isn't that important. But if so, it depends on the specific neutral. For slow ones (those that don't reach the archer in two turns), position them at the bottom. For those that reach in two turns, but not faster, it doesn't matter: they usually skip their turn to avoid being brutally shot down by a direct arrow. For those that reach in one turn, position them at the top: then, during the hero's first turn and with the "haste" skill, you can try to shoot before their turn. However, with the "slow" skill, it's still better to position them at the bottom, and if there are no fast units, there's no difference. The previous comment about the ballista was correct. When fighting against archers, position them at the top. And it's still best to acquire the "tactics" skill as soon as possible, as the terrain is just as important in this matter.