It's simpler and more prosaic. Archers shoot at centaurs because they immediately charge forward. That's why I always separate the single centaurs when breaking through archer formations and send them forward immediately. The other day, playing as Fortress on map 111, I got boots of speed for 29 gold. I had elves with a loss of 4 ogres and 5 centaurs, and armies with starting heroes, without ambushes or magic. I didn't take the elves into the battle. Too bad my opponent chose Rest. :(