yes, heal is very powerful spell

I would rather have cards that eliminate those specific circumstances, instead of a generic counterspell. Something like the following would be a lot more appropriate in this game:

Insurer
Cost: X4
Attack 1, Health 2, Move 4, Vision 4, Range 0

If Insurer's attack causes a creature to have 0 or less health, that creature cannot be the target of any spells, enchantments, or special abilities.

(Alternatively, said ability can be in Enchant Creature form as well)

Basically, here is the sticking point for me. A counterspell happens after a spell is cast. But even dew mogi is a preemptive card. The idea being "if you cast a spell, you know this is going to happen." I'm all about preemptive denials of spell casting. But a counterspell will be so necessary that everyone will have 3 in their deck - and forcing people to hedge on the high likelihood one is in your hand at any time severely undermines the importance of spells in a deck. I'd rather encourage spells to be cast out of strategy rather than paranoia.

Also, I feel like the back and forth mechanic of this game just doesn't lend itself as easily to these types of denial cards. Negate kind of works because it's entities. But even then, you can do it for some time after the card has been in play.

But you also can't have more than 3. Mogi Nest, in theory at least, gives you an infinite number of Dew Mogis.