Mulligans
General Mulligans
In most matchups you'll want to start with cheap removal cards, and Blood Razor is generally great to keep if you can curve up to it well enough.
Aggro Mulligans
Like in general, just cheap removal cards to survive the aggressive deck's early game. Shield Block is especially good against Pirate Warrior since that's a deck that tends to deal it's damage then get burnt out, armour being very good against weapons.
Midrange Mulligans
Similar to playing against aggro, except you can afford to keep some more expensive options such as Blood Razor or the Whirlwind-Sleep with the Fishes combo.
Combo Mulligans
It depends on the combo deck - Shield Block is good against Alexstrasza based decks, and Dead Man's Hand can save you from mill rogue. Against combo generally though you need to kill your opponent very quickly, which this deck naturally isn't good at meaning you should almost always keep clocks like Ravaging Ghoul or Arcane Giant.
Control Mulligans
Try to keep mostly removal that can kill large minions, except for perhaps Slam which draws you a card anyway. That and Battle Rage are fine to cycle through your deck - you'll want to try to start playing Arcane Giants before your opponent gets to their late-game, which is why it's acceptable to keep Giant as well, which you'll generally just want to replace if there's something else you're lacking.
Control the game with classic control warrior cards, then combo Arcane Giants with cards to copy them for the win.