Mulligans
General Mulligans
Generally, you want to ramp and complete your quest with oaken summons+fatespinners to clear whatever your opponent plays.
Aggro Mulligans
Against aggro, depending on what kind, you have to be prepared to maybe give up on the quest and just survive.
Usually cards like Spreading Plague will give you a lot of time, but again if you trust that you can ramp and complete the quest in the process, why not!