Mulligans
General Mulligans
Northshire: cause it's OP
Potion of Madness: cause it's OP
Netherspite: 2 mana 1,3 battlecry draw a synergy card is never a bad 2 drop
Shadow word pain: e-z removal spell
Kabal Courier: this deck has pretty weak 3 drops, at least on the curve. Your only other 3 cost card is one that serves a better purpose in the mid or late game, so a 2,2 is pretty good on 3 for this deck, especially as you can use it to discover a cool tool for later.
Duskbreaker: because it's always an aggro meta, and this is busted against aggro.
Use synergy with dragons to stomp on your opponent!
Early Game:
Use Northshire Cleric, Potion of Madness, and Shadow Word Pain to control the early game
Use Netherspite, Shadow Visions, and Courier to discover more options for later
Mid Game:
Use Duskbreaker to clear an unfavorable board, and Blackwing and Drakonid to establish a presence
Late Game:
Dragonfire and Anduin when you need to clear the board, most of your stuff lives anyway. Wyrmguarding on 7 is an easy way to stop aggro in their tracks. Entomb anything you don’t want your opponent to have, stealing win conditions is always fun. Use Primordial Drakes as backup clears and taunts for if your opponent floods the board with little guys. Use Acolyte in combination with a pain to simultaneously remove and generate threats. And use Ysera and any other cool stuff you’ve taken/discovered to win.
Replacements:
If you have either the dust or the cards, some good replacements would be:
Harrison Jones instead of Blackwing Corruptor
Curious Glimmeroots instead of Kabal Couriers
If you don’t have the dust for some of the more expensive cards, some replacement options would be:
Power Word Shield instead of Shadow Visions
Shadow Word Death instead of Twilight Acolyte