Mulligans
General Mulligans
Allways mulligan for Prince Keleseth unless you have Fire Fly and Witchwood Piper in your hand, because with Witchwood Piper will allways draw Prince Keleseth if you allready drawed Fire Fly.
Aggro Mulligans
Try to get Prince Keleseth and Fire Fly but keep Tar Creeper and maybe Stonehill Defender.
Control Mulligans
Mulligan for Prince Keleseth but keep Fire Fly and Witchwood Piper if they are both in your hand, also keep a Spiteful Summoner if you can.
Use Witchwood Piper to draw Fire Fly and Prince Keleseth.
Use Leyline Manipulator to reduce cost of the cards you get with : Fire Fly Igneous Elemental Stonehill Defender Steam Surger Servant of Kalimos.
Keep some elemental with reduced costs for Ozruk.
Stay alive with the help of Frost Lich Jaina and Baron Geddon.
Have fun !