For easier balance, it could have only its ATK based on your life total and not its HP (I mentioned a card like this before in a Beast series idea in Card Theory section). It might be a good idea to have a cap so SS/SoD can't drop an enormous guy, or work the formula so HP over 100 gives diminishing returns. I'd also think it would be better to make it not be too useless if you have low HP.
Example of diminish returns: 3+(sqrt(HP)/2)
400 HP => 13 ATK
256 HP => 11 ATK
144 HP => 9 ATK
100 HP => 8 ATK
64 HP => 7 ATK
36 HP => 6 ATK
4 HP => 4 ATK