In a battle of 100 vs. 100, speed is even more important than in a 1 vs. 1 fight. Whoever strikes first will immediately kill a certain number of monsters, meaning the counterattack will be with fewer than a hundred units. In a 100 vs. 100 archmage battle, they will tear through everyone, except for phoenixes, if you take 200 of them (because their growth rate is twice as high :).
That's exactly what I'm saying: an objective assessment of any creature shouldn't be based on who strikes first, because that doesn't make any sense. If a unit has turn priority, why should it attack second?