I've playtested this deck with many people, and it actually is a pretty potent killing machine. In teams matches it is particularly deadly, since it basically bypasses life gain. So far, only black or artifact based decks prove to be good counters to this deck.
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Please note I've made a change to the deck because of a grievous error. You cannot use Stuffy Doll with any poisonous ability because it must do combat damage to deal poison counters. However, Marsh Viper and Sabertooth Cobra can use Viridian Longbow to deal poison counters because they do not actually have "poisonous".
I would suggest adding more cards that give trample. "Wildsize" from Guildpact gives +2/+2 and trample plus draw a card for only 3 mana.