What they said.
Even a Virtue going in melee with her buffs can work very well, though Air spells alone are unlikely to be sufficient (the only point is to get experience faster, I believe).
On the contrary, the better the independents, and the harder the research, the better the Virtue does: if independents are on level 3, then any small army will take them down, making fighting Pretenders useless in expansion. And the Virtue absolutely loves the Knights.