Mulligans
General Mulligans
You want to get at least one type of removal in your hand to counter his first big threat. Alexstaza is helpful to get him to low health early, but not a neccesarily important mulligan.
This is an easy deck to beat Nightbane on normal and heroic. It uses cheap removal and big minions to simply out power him. Obviously you don’t need every single legendary on the list, just substitute some of them for other high cost minions. You want to mulligan for at least one sort of early removal to kill/stall his first turn big minion and to get you a massive lead on the board with 10 mana