Tactics for playing as the Sylvan faction.

I saw somewhere that it's good for Elves to level up magic on a Phantom (summoning, if I'm not mistaken), but then you lose СА or another useful skill. Is it worth doing? And if so, which skills will be useful for this magic?

Well, if you're playing as Dyraell (she has summoning from the start, which allows you to learn Phantom, which you otherwise have to find somewhere). Playing other heroes with a focus on summoning or spellpower-wisdom skills doesn't seem very practical.