Mulligans
General Mulligans
Generally you want your early drops in the beginning, for gaining quick board tempo and potential swinh turns with kanrethad. It's easier to outcast felosophy if you mulligan it so it gives additional stats as well.
Aggro Mulligans
Agaibst aggro prioritise your soul shard generators so you can consistently activate void drinkers and shadowlight scholats, and get extra heal.
Combo Mulligans
Maximum aggro, you want to pile up pressure early on and use giants asap to close the game.
If you already have a 1/2 drop it's good to keep hand of guldan for your card draw so you can keep mounting pressure.