Two possible options to consider for Maw to tone it down - rather than just nerf
Third trait (Devour) only become active AFTER the spell has been cast
This would prevent games being won/lost after a lucky first turn skull match.
After the one-shot spell is cast, that the troop can/must fill up with Mana again
This would make the one-shot spell troop become a potential blocker for a short time (though obviously makes the secondary element of Maw’s spell a lot less useful). Once again this would give the other player a chance to recover after the potentially game changing effect of having a troop devoured has occurred.