So I thought zanz didn't want any more insta-kill effects in the game. Then I see this.
Think of it like Fire Shield.
If the health is low, it will be killed faster. More emphasis on probability here, since every round has the exact same chance, but a lower health creature will turn before a higher health on more occasions. In this case, it is uncounterable by growth and healing, just delayed.
I really like this card now. It's a shame it still has the name skull shield, I would get rid of that and name it something without the 'meh' reputation. It didn't even lose it's synergy with aflatoxin. Impressive Zanz.
Death effects almost seem like overkill, but vulture, soul catcher and boneyard don't see that much competitive play. And this rewards high Health (like most cards should). Good card. Good work.