Mulligans
General Mulligans
Keleseth is always good. You want your spiteful and floop, because that gives you a third spiteful right there. Play your high tempo minions, and then just win the game with 2-3 tyrantus.
Aggro Mulligans
Freeze, Taunts, and Mind Control. Mind Breaker techs against odd paladin.
Control Mulligans
Get your snowball minions, your spitefuls, and your spiteful value cards. Smack them around with big 10 drops. Silence their taunts.
Play Spiteful Summoner on 7, then you can either play Floop to get another tyrantus for 3 cheaper than that other one or Spray some gloop to summon two 4/4 and another tyrantus. Other than that, normal aggro/midrange play.