Mulligans
General Mulligans
Necrium blade is your must keep in all scenarios. Devilsaur egg, sudden betrayal, and sap are also good to prevent loosing the tempo of the game. Meat wagon, blightnozzle crawler, assassinate and cursed castaway are also good to keep if you already have a good starting hand to prevent drawing your high cost minions. Zilliax is always good in any play, and master oakheart is a great play on its turn but also looses value as the game goes on. Only keep cold blood if you already have a meat wagon on hand.
Aggro Mulligans
I havent maked any statistics, but im pretty sure aggro is an unfavorable matchup, still i managed to win a lot of games vs aggro due to the deck survavility (Zilliax, Deranged doctor, Shadowblade). i would say it depends a lot of your draws.
Midrange Mulligans
Midrange decks are very easy to beat due to the single minion removals of the deck and the high tempo swings it can provide.
Combo Mulligans
I dont remember wining any game to a combo, the deck is just too slow for it. Maybe a good hand and great draws may be enough to beat combo. Freeze combo mage is your worst nightmare when playing this deck.
Control Mulligans
I have found control very easy to defeat, most decks cant do anything to a board wich summons another board on death, even Phsichic scream or Vanish are enough to stop the deck.
Recruit Rogue, a deck i previously posted on Witchwood as a very pretentious deck and kinda memey. This time thanks to Boomsday deathrattle sinergies for rogue i managed to make an actually working deck; i properly started ranking up with it. Now, it is not a high tier deck, but as an off meta works really very well.
Meat Wagon; the card itself can pull out of your deck a Devilsaur Egg to start combing with deathrattles. But when you cast Cold Blood on it, it becomes 5 attack, wich means it can pull better minions from your deck upon death. (On 5 attack it can recruit Blightnozzle Crawler, Zilliax, Mechanical Whelp, Silver Vanguard, Grand Archivist.
Devilsaur Egg is your cheapest deathrattle synergy, it can start harassing your opponents very early but its also your best way to stop aggro from spreading the board early on. In this deck Cold Blood is mainly meant to be casted on Meat Wagon, but you can use it on the egg if needed.
Zilliax is also a card that can make Meat Wagon recruit better minions, and it can actually be played on curve easily. It is also a great card by its own and can provide a lot of survavility wich both rogue and the deck itself may need. The deck also runs a lot of mechs giving extra sinergy to the card. You can also play Cold Blood on it if you need that extra healing.
Silver Vanguard has a great deathrattle sinergy as the five minions it can recruit are all very powerfull. The card itself is not that great by its cost, but due to deathrattle activators and the fact that most of the time you will be recruiting her from your deck it adds a lot of power to the deck.
Grand Archivist on rogue. It sounds weird, but look at the spells on the deck, all of them are great to be casted by him (except from cold blood). Sap and Assassinate will most of the time turn the table to your side, but are also very good at stoping your enemy atacking your face. And if you think it can waste your Necrium Vial let me say that never happened to me since i started playing the deck, mainly because Devilsaur Egg and Mechanical Whelp are very easy to stick on the board. i initially ran a copy of Sprint, but due to the recruit style of the deck it ended up being self defeating.
Cursed Castaway is meant to pull a Cold Blood from your deck in case you didnt drew any from your deck, but its also good to use her to draw your second one to prevent Grand Archivist from casting it.
Master Oakheart Is a very strong card sinergising with the whole deck. It is meant to recruit a Meat Wagon, a Mechanical Whelp and a Silver Vanguard. Still having both Blightnozzle Crawler and Zilliax makes it more reliable. It is a card you want to play as soon as posible in turn 9 (or 8 + coin), but when i know it will recruit a meatwagon, and i have a Cold Blood on hand i like to wait untill turn 10 (9 + coin) to ensure the wagon being buffed.
Edit: Replaced a copy of Blightnozzle Crawler and The Lich King for two copies of Charged Devilsaur.
Removing a copy of Blightnozzle Crawler adds a lot of consistency to the main combo (Meat Wagon + Cold Blood). And i removed lich king because i was never using its cards anyways, i think a charge minion can be still good vs aggro, but defenitively better vs control/combo.