Discussion about Sylvan (Wood Elves).

Basically, yes, but there's also a small issue (what if the hero moves before the unicorns?) – resistance will save them (at least it will try!).

Well, if the hero moves before the unicorns, then it's clear what to do with them next – attack.
P.S.: When playing against Necropolis, I always take unicorns with resistance. I still mainly use mass light spells with my hero (that's why I don't take light creatures). And with Druids, I prefer to use magic attacks (or just shoot) rather than using Stone Skin on two units (light unicorns and someone else).