What kind of magic do you like the most?

By the way, the Dominate spell works great on Phoenixes, if I'm not mistaken.

You're mistaken ;) An Elemental is an Elemental.

And how are you going to win a hopeless battle with Summon when facing more or less substantial armies? It's possible in combination with other spells, no doubt, but with just Summon alone?

What do you mean by "substantial"? 100 dragons? Well, yes, then it's problematic. However, this situation is only possible in single-player scenarios, and even then, there's always a way to avoid letting it drag on.

I suggest the simplest option: just ignore them and cast Darkness on more suitable targets.

And you'll watch as a bunch of zombies slowly and not very effectively beat up a bunch of skeletons, or vice versa... Meanwhile, the Phoenix, together with the remaining enemy troops, will perfectly decide the outcome of the battle ;)
It's pointless to talk about the neutral school because then you'd have to include all sorts of skills like "Banshee's Cry" and "Enchanted Arrow."