Well there are so many more factors than just what you have listed, but for Azumarill with leftovers and aqua ring, switching into an offensive grass type such as roserade would be smart, and just OHKO it with leaf storm.
For Serperior that is fast, a Pokemon with taunt, or even better magic bounce, should do the trick, just because Serperior has terrible attack stats.
For Roselia, (the move is called leech seed by the way), an offensive grass type that carries hidden power ice, fire, or psychic should force the switch, vileplume would be good because it can't get poisoned, or exeggutor, because of its stab psyshock. Actually Exeggutor with psyshock is the better counter.