The four most powerful spells I can think of are Lightning, Meteor, Hailstorm, and Word of Power.
Word of Power's strength comes from the army against the user
Meteor is single target full power
Hailstorm is better for melee, because its multi strikes miss the farther it goes. But at melee it goes full blast, all hail the Monk.
Lightning, however, possibly surpasses them all. I'm not sure, I need to test it as it is multi hits. Those hits count on the target's movements. If the only one to hit is ranged, they will stand still. That means the lightning will do full damage. Not to mention there's no protection against it as it goes through Rebound.
The Dark Mistress is pure proof we don't want this thing in its current state.
She has a high health, making her stand longer. And she's so fast that she outmoves most melee units. The Fairy can't outrun, she has too many spells to cast. The Roc's ability to avoid melee is even better than the Dark Mistress' speed to out run melee fighters.
And also, the weakness of a flying creature should only be with this ability. It limits too many other creatures/heroes if it is for all flying things. Its not like they have an advantage like this.
Even not as a flying creature, this unit seems way to powerful as a melee. He has high health and is pretty quick on his strikes. He has an average defense, not a poor one. His armor may be low, but protect will do more than take care of that. And a medium-high attack? Yikes. I think you just put the Horned Reaper out of buisness.