Green usually doesn't get lifelink. The only time it does is if the card also costs white mana or has a white or black activation cost for the ability. So that seems a bit out of place here. As for the black effect, I'm not sure this actually works. I was experimenting with something like this recently, and I like where you're going with this, but I don't think it's quite that easy. I would recommend "Put X -1/-1 counters on target creature, where X is half that creature's toughness, rounded down." If you did that though, and even if you don't, this definitely isn't an ability you would see on a common, or even an uncommon. It would have to be rare or mythic because it's a very powerful ability. It should probably cost at least two black mana plus a colorless or two, which makes me wonder if this card should just be black, as you suggest. If you did shift it to black, you could keep lifelink, also.