Mulligans
General Mulligans
Always keep Northshire Cleric, Power Word: Shield, Radiant Elemental.
If you get Inner Fire or Divine Shield, keep one of them as you need these cards for combo.
Keep Netherspite Historian if you have another dragon in hand. If you are starting second, probably you can consider to keep Twilight Drake as you have more cards in hand going second. Twilight Drake can be the highest health minion you can target your combo with.
Based on the wild combo priest with wild cards substituted with dragon synergies to ladder up from rank 15-5.