Interesting idea. Obvious synergy with Parasite/Bloodsucker. I'm not sure whether or not that would be OP. 5 Bloodsuckers all boosting it every turn and it would get pretty ferocious, but that's a lot of cards to put together obviously. I don't understand the healing mechanism. It doesn't take damage from poison counters, AND it heals from them? So if it's got a bunch of counters on it, you have to one-turn kill it? If that's the case, I think both versions are OP for their high health. This thing would be incredibly hard to kill.
Also, when does healing occur? Seems to me it would make the most sense for healing to occur pre-attack, so it would heal, then take damage from Fire Shield.