Hearthpwn

This is the deck that I play post 2nd wing Naxxaramas.

The purpose of this deck is to build armor, clear the enemies board and basically outlast him.

This also means that you might have to be very patient with your threats. The games I lose mostly come from being to eager and not getting value from a threat.

I have around 65% winrate with this deck on ladder, and it fares very well in tournaments as well.

Mulligans:

These are only guidelines, some of them I'm not even sure about, but they will provide a good stepping stone. 

Warlock:
If we expect zoo ( when in doubt I always mull for zoo ) we want to keep early game minions and war axe. A single Sludge Belcher ( which destroys zoo ) or a brawl can also be a keep on an otherwise sketchy hand. Acolyte of pain is extremely good here for card draw  and board control. Don't be afraid to use a slam to remove a minion without card draw. Execute is less good here than in other matchups.

If we expect handlock execute and black knight becomes extremely good. I'll keep shield slam, executes and a war axe. We are afraid if the handlock plays with eggs. He has a limited amount of threats and we should be able to handle them, but the eggs can be hard to handle if we don't have an early creature.

In general we should win over zoo and have 50/50 vs. handlock.

Warrior:

If we know it is an aggro warrior, mulligan as if it is zoo, otherwise try to get as much early game pressure as possible, and consider keeping a loatheb.

Rogue:

If we assume miracle we mulligan for every taunt we have, we need to keep armoring up and keep taunts on the table. We have to be careful that we don't overcommit to the board to give them too good blade flurries and Fan of Knives. Sludge Belcher is our best friend, and a well timed Loatheb might swing the tides in your direction.

Overall keep your armor up and their board clear and you should be good.

If we assume cancer rogue we mulligan and as with zoo.

There are usually no targets for The Black Knight so just throw it on the board.

Paladin:

Whirlwind is the boss. Sludge Belcher is again an mvp preventing the paladins strong board clears. Play it slow don't overcommit to the board and you should be good. Shield slamming you own sylvanas to steal a Tirion might save the day.

Shaman:

Shaman is a tough one but can be won. Baron Geddon is our MVP.

Keep all the axes and sludge belchers you can. Acolyte is your friend, and Cruel Taskmaster nice as well.

Don't allow him to flood the board with totems. Try and bait hexes before you play rag. The Black Knight usually don't have great targets so it's fine to take a Stoneclaw totem with you. But if you are ahead, slow play him in case of Defender of Argus, or Al'akir.

Druid:

Against a token Druid I think we're favored and should mull for removal and board control. An acolyte also helps. As long as we can keep their board clear we'll win in time.

Note that Loatheb shuts down their combo for a turn, so he can be used to set up board control in a more shifty situation.

Against taunt Druid I feel we'll just have to try to keep up, getting maximum value from our big threats. Mulligan for acolyte, and execute. I don't think we have a very good matchup here. But we should be able to win about 50% of the time.

Priest:

This is a very weird matchup, and one in which we're not particularly favored. I'd mulligan for early board control and otherwise play it extremely slow and reactive.

Black Knight can be saved for Sludge Belcher But can also serve as an annoying 4 power minion. Priests sometimes have a lot of burst in double mind blast + Holy Fire, so try and stay above 15 HP at all times.

Hunter:

I feel like we only should lose this matchup if we misplay or draw extremely bad. Keep armor up. Mulligan for Armorsmith, War Axe, whirlwind, slam.

Do not overcommit to the board to limit the damage that unleash the hounds can do.

If you're playing a midrange hunter, try to save The Black Knight for misha or something targeted by Houndmaster.

Mage:

If we feel it is an aggro mage we mulligan as vs. zoo, we just need to stabilize before we hit a too low life total. Be aware that taunts does not save us as much HP here, so armoring up is much more vital.