Deathrattle Demon Hunter – #23 Legend (mediocer) – Barrens Caverns

General Mulligans

Skull of Gul'dan is always a keep in outcast position (also a keep if not in outcast but cards left to it can be played before turn 6.)

Death Speaker is always a keep (except vs  Druid,  Shaman and Mage).

Kazakus always a keep.

Tuskpiercer is a good keep.

Razorfen Beastmaster and Razorboar are good keeps with a deathrattle or a Tuskpiercer  (can be kept without a synergy card but I'm not sure if that's optimal).

Wandmaker is a decent keep.

Illidari Studies is a decent keep.

Devouring Ectoplasm is a fine keep on the play.

Felrattler is a good keep vs Hunter 

Aldrachi Warblades is a keep vs face decks (e.g Hunter, Shaman).

 Flex Slots:

Wandmakers can be swapped with Sneaky Delinquents as this build is a bit starved of Deathrattles.