Hearthpwn

Seance really shines, and is a win condition on its own through baiting concedes. Best target is almost always Amara, Warden of Hope, but Archbishop Benedictus is also very solid. You can "bounce" a Duskbreaker or Dragonmaw Scorcher for extra clears, or add another DR minion to complete the quest. Getting a solid minion off your opponent like Rag or King Crush then Psychic Screaming their board is also pretty demoralising. Unlike almost any other deck out there, you WANT them to know what's in your hand.

-Awaken the Makers allows you to outlast aggro and can get you out of range of some combos. With 4 draw minions, 3 that generate cards, and a taunt heal you have enough options to get the quest done quickly whilst generating extra value for Princess Talanji 

-Bone Drake has a good chance to be drawn from Dead Ringer to activate your Duskbreaker, leaves something for Princess Talanji and is a solid body.

-Weaponized Pinata is fun, and you could run two over Rotten Applebaum if you wanted. Quest and Princess power with enough random to make it interesting.

-Twilight's Call gets the quest done quicker, and can get you extra dragon and legendary cards or a bit of draw, watch your hand size.

-Stonehill Defender isn't the strongest taunt out there, but can slow them down and makes your Princess turn a solid wall that a lot of decks will struggle to get through, also a fair chance of finding a DR minion or even Primordial Drake to help activate Duskbreaker. 

-Princess Talanji just makes big boards, fed by the above card choices. Be aware she will pull Amara, Warden of Hope, but an 8/8 taunt isn't to be sneezed at and this board alone gets you that magical 15 damage if it stays up. Always plan what needs to be played for the battle cry instead and try not to be too greedy. You have up to 7 ways to generate a new card outside of the Bishop, but can only get 6 on board at once anyway.

-Archbishop Benedictus can help the Princess, but is also great for stealing Death Knight and Hero cards. Your opponent knows what you now have a copy (or two) of, and if their win condition is still in the deck you can get some concedes. Playing 2x Zul'jin is most satisfying after Mind Blast, and a Shudderwock can be hilarious. Enjoy the memes!

-Duskbreaker, Mass Hysteria and Psychic Scream are all solid clears, and Primordial Drake has the obvious synergy plus taunt as well. Feel free to scream then bishop to get your hands on some choice minions or fill pally/spell hunter decks with their useless tokens.

-Dragonmaw Scorcher is great against odd pally and not a bad play into Mass Hysteria a lot of the time. 

-Gluttonous Ooze can also be a Seance target against weapon heavy decks. Along with Skulking Geist and Mojomaster Zihi it forms the core counter set you may or may not choose to deploy.

-Alexstrasza has dragon synergy and gets them within kill range (or saves you in a pinch). Used before a Mind Blast and Voidform turn or after a big Princess Talanji, this card is your best bet at beating combos and ending the game around turn 10.

-Value! This deck creates a huge amount of Health and Minions with enough clears to help you get back on top and just win the attrition game. Seance, Archbishop Benedictus and Amara, Warden of Hope means you will outlast fatigue, whilst Princess Talanji and Alexstrasza help you never have to get there. 

-Combo! Alexstrasza into 2 x Mind Blast as Shadowreaper Anduin for 16 damage will win you a lot of games and is generally what you are aiming for in hand by turn 8, 9, 10.  A big Princess Talanji board into Alexstrasza is a nasty surprise as well. 

-Concedes! Beyond this the sky is the limit, in typical Priest fashion you can use Seance to:
-beat them with their own minions
-clear, clear, clear and more clear their board
-demoralise them into conceding with 120 health gain or 60 card decks; letting them know you have all the answers for a long game will make a lot of ladder climbers simply quit rather than lose in 15 minutes time anyway

Your plan will be similar for the first 5-6 turns against almost any other deck, try to get your deathrattle minions out and hope for a big clear to negate their early game. Always try to Seance Amara, Warden of Hope from turn 7 and then just out value or hope for your game finishers against combo decks. 

Generally, Combo decks mean you want to use Twilight's Call for draw if possible and hold off on Archbishop Benedictus to get to your own combo faster. Control decks you want to play him early and save a Seance for him. Aggro decks you want to Seance a Duskbreaker or Dragonmaw Scorcher if you can.

Your general Mulligan is Awaken the Makers, Duskbreaker > Dead Ringer > Loot Hoarder > Stonehill Defender > Mass Hysteria with coin > a second dragon.

The #1 tip I can give is don't be too greedy; almost all your minions cycle or add to your hand, Princess Talanji can obviously thin this out but it is very easy to be sitting on 9 cards most of the game waiting for the last piece of the puzzle and hoping you don't burn it.

Know if and when to play your legendary cards and Seance. Any class with big clears is just waiting for your Princess turn if you don't bait them first. Playing Alex or Ben early can really ruin your game. 

Know how much damage you can take before healing back to 40, let them extend into Mass Hysteria or Shadowreaper Anduin if you have Amara in hand.

Seance your Amara, Warden of Hope (or twice on turn 9) and then use her with Princess Talanji to make a huge board. Play her same turn as Mass Hysteria for a pretty solid clear. Most times healing to 40 health once is enough, but the threat of doing it again is worth having and nobody expects you to drop at least 15/13 of stats for 8 mana.

Use Stonehill Defender wisely, sure you usually want huge taunts with Talnji but sometimes the smaller cheaper option will keep you alive long enough to actually play her. Look out for Deathrattle minions (as Priest these come up often) to help complete the quest.

Play around Twilight's Call if you can afford to hold off on your 2 drop draws for the chance at extra dragon and legendary cards. 

Zilliax is magnetic and you have two mechs in your deck, is often a great Seance in a pinch. 

Seance can be played with every other card in your base deck, grab an enemy minion before you Scream or Anduin it, play Talanji twice if you already played Ben, pit Lich King or Ragnaros against themselves..

Overall, the spells I miss most are a second Psychic Scream, Mass Dispel and Spirit Lash/Shadow Visions. Another Primordial Drake,  Dragonmaw Scorcher, Rotten Applebaum, Weaponized Pinata, draw, or silence would be nice too. I'd start with these if you want to replace anything.  

Within the Deathrattle/Dragon shell there is some room to try Swamp Dragon Egg for the delayed value game. Or get Prophet Velen and Gilded Gargoyle for the 20 damage combo.  Go more Thief with Cloning Device or Crystalline Oracle. Even Spirit of the Dead would have some synergy. 

Besides the fun and value factor, Archbishop Benedictus is there to win the fatigue game against Warriors and Mages. Not particularly strong against Hunter or Paladin, swap for a second Psychic Scream or cheaper memes with a Spirit of the Dead.

Princess Talanji can be hit and miss, but an Amara, Warden of Hope with just another taunt or big dragon is a decent swing turn against a lot of decks with no clears, great lead into Alexstrasza. Nothing really comparable, maybe Elise the Trailblazer 

Shadowreaper Anduin is your big board clear and win condition, enjoy him for just a little while longer or throw in a Holy Fire.