I think it'll do more harm to Lava Destroyers seeing that such decks would need to rely on more than the mark to power the skill.
That said, rather than having a set defense, why not have its defense rise up by 1 every 5 turns or so, like lava hardening? It already seems like the shield needs to grow seeing that it takes up quanta any way haha It'd have to raise the price a little more, though, so that it can't be played that early on in the game.