I think only the Massive Dragon should get a buff.
I mean you upgrade it and get 30hp... who needs 30hp? And noo.. you don't you use gravity pull on them, armagios are for that; they cost less, have less attack to lose, and you don't waste a spell card to use it on them.
Anyways Massive Dragons hp should be lowered down, attack should be pulled up or just make it come in play with momentum.