TL;DR: Remove 2 ATK instead of one for both versions and make it a permanent.
In MTG, you tend to not mitigate threats as to utterly remove it.
MTG is not a good way to balance this card. Why? Look below:
The most common way of reducing ATK (currently in standard) is
hydrosurge. That gives a card minus 5 for 1 turn.
Pin to the earth is a more permanent effect, giving a creature -6 permanently.
A card that's very similar to your card, or the most similar, is
cumber stone.These are bad cards. All the better cards also reduce toughness, not ATK.
So we just have to balance it in ETG standards.
So I'm just gonna go on gut and say make it like cumber stone. Just better.