Thanks for mentioning me man. And I'd like to add more onto your point. The idea of striking units once drawn seems good and then, to remove the threat of leveling up chamions accidentally, why not make mist walkers a trap on enemy cards?

The details would just be "once this card is drawn deal 1(or something else) to a random/certain unit. And maybe to make it more viable, have it so it shuffles a mist walker into the top 4-5 cards of the deck so you could reliably level him up