imagine an oty or scarab (pharaoh fits with time) that can eat the whole field by itself, in one turn if you have enough gravity quantum.
that's why the creature get -0/-1 (this way in a devour case, only it's atk will rise with the devour)
but that's why i'm thinking if it could be add something like
this card can be used on the selected creature only once per turn
even if the card can be played once per turn it doesnt matter,
suppose if you have a 4/9 oty, you play the card, gain this effect:
Target creature can use it's skill one more time this turn.
It gains -0/-1
even if the -0/-1 stacks, you can easily eat 5 creatures (if your opponent has them) and become a 9/9 creature, while your opponents field is clean.
Not saying this is OP, but it could use a bit of tweaking

But for oty's ability it's kinda OP in my opinion.
But the way you explained it, it seemed to me as the creature only gets 1 time -0/-1, not every time they use their skill/ability (you could adjust that with some info).
greetz.