Best level 6 unit.

I'll provide my reasoning.
If, during a battle, two units of troops have been reduced almost to "0", meaning they've been thinned out, and the opponent has someone in secondary sites and/or Castles to replenish their army—that is where the question arises as to who exactly should be used to replenish the army.

In this case, I agree with you, but as I understand it, Ley suggested specifically creating an army of 7 units from different Castles (all 7s, 6s, etc.), whereas in the scheme you described, the situation is different—naturally, instead of 10 troglodytes, for example, it's better to take a couple of efreets.
Ley, with negative morale, my units often skip their turn, but positive morale works much less frequently (two or even three times less).
The efreet shield is not considered magic (no idea why) and works even on dragons. And its strength, if my memory serves me correctly, doesn't depend on anything at all—it is a constant value and independent of the hero.