The only thing you want to think about at mulligan is to curve. Make sure you have a 1-cost and a 3-cost and hope to get Keleseth, any high cost minion is send back to the deck. you only keep Shadowstep if Keleseth is also on your opening hand. In case you get multiple 1-cost or 3-cost minions choose the better one for the matchup. Argent Squire is better most of the time, but in some cases you preffer the 2/1. For 3-cost Emperor Cobra and Mindbreaker are always better than Deathspeaker and Plague Scientist. Keep Mindbreaker if you think your opponent is playing Odd/Even decks.
This deck is a remake of a deck i posted back in witchwood; now with cards from the new sets, and thanks to the recent nerfs i finally make this deck work again, and better than before. As one can guess by the title, this deck rovolves around poisonus minions, but noly relies on them. The main point of the deck is to play this valuable minions (such as poisonus) and then keeping them alive thanks to your other minions.
Prince keleseth synergises with almost all minions in the deck, since the main point is to keep your minions alive. Shadowstep is on the deck almost only to highroll double keleseth on turn 2, but still has good sinergy with other cards in the deck.
All 1-cost minions are in the deck because keleseth, but also because you want to play a safe 3-cost on turn 3 without letting it die instantly. Argent Squire and Southsea Deckhand can also have good synergy with Plague Scientist later on the game, and Glacial Shard battlecrie will always be useful.
On turn 3 you mostly want to play Emperor Cobra or Mindbreaker so they can stick to the board as soon as posible. Mindbreaker is a very great minion to play against Baku/Genn decks as you force them to actually play cards. Although i previously said that the point of this deck is to keep those minions alive, having your minions die is part of the game, what really matters is to play tempo, so have no fear in making value trades or letting your minions die if you cant protect them, you will have better minions later and your minion protection cards will always be useful. Deathspeaker and Plague Scientist
are obviously meant to be played later on the game, but still playing
them on turn 3 is better than nothing, specially if they got buffed by
keleseth. Deathspeaker works very well on minions with poisonus as they can kill every minion, no matter how much health or atack they have, so trading them on big minions while taking no damage is always great, even if your opponent can destroy it in they turn. But also playing him on any big body is always great, even playing it on a 1-2 drop at tempo is good. Plague Scientist has extra sinergy with the deck, as most of the minions are big health ones, Mindbreaker and Deathspeaker themselves are great candidates for its battlecry.
Master of Disguise is the card you want to play on turn 4; it adds an okay body to the board but most importantly prevents one of your minions from dying on your opponents turn (specially Mindbreaker, Emperor Cobra, Venomancer or Snapjaw Shellfighter ). Fal'dorei Strider doesnt have that much sinergy with the deck, but a great 4-cost from rogue after all (as i didnt find that much good 4-costs). Elven Minstrel is weak as a 4-cost minion, it is even a weak card for a 3-cost minion, but the draw it provides is too strong to be ignored, still youll preffer to play a 3-cost on turn 4 that him.
Snapjaw Shellfighter is basically the card that make this deck great again, it oversynegises with the deck. Not only does it help with trading your poisonuns minions while keeping them alive, but also protect your minions from your opponent ones, and most importantly from your opponent AoE, as it takes all the damage from the adyacent minions. Also playing Deathspeaker on her on a turn will devastate your opponent, as it will triplicate deathspeakers battlecry. Venomancer is just another of your poisonus minions, but also your strongest one. Despite what it seems, you can safely play on turn 5, on that turn your opponents will hardly remove it without wasting they turn.
Cursed Castaway is an okay card that simply works in the deck, her rush mechanic works well at defending your important minions and also synergise with keleseths buff, her deathrattle will give you either a Plague Scientist or a Elven Minstrel (unless you already drawn all of them, wich may be unfortunate). Nerubian Unraveler is a recent addition to the deeck that i copied from recent hunter decks, but im still testing it. Gurubashi Hypemon is that card that every time i look at the deck list i think i should remove, but every time i play it convinces me otherwise. I cant even say how you should play it, just play when you dont know what to do and expect something good to happen. Marin the Fox is the ultimate card of the deck, whenever you have this poisonus minion with nothing good to kill just play marin for instant value. If you have marin on hand you often want to attemp to prepare that situation. Also you can just play it and let the treasure on board so enemy AoEs damage it, this also makes the treasure vulnerable to Cursed Castaway and Plague Scientist. Just be careful with opponents that can take advantage of that, such as paladin buffs, shaman evolutions, priest combos (or psychic scream) among others.