I really don’t know about merciless. 65 Atk isn’t good. And if you patch it up with choice band, you lose the ability to use baneful bunker, probably your main source of poison.
It’s a great ability. But poison seems like a really gimmicky horse to hitch your entire wagon to, especially on something as weak as this. It’s worse than sniper Kingdra because 1) you can’t control the criterion with which your ability activates and 2) Kingdra is good because criticals let him Spam Draco, where there is no such attack here, so your only niche is on defensive boosting Pokémon.