You can use either Ghostly Strike or Walk the Plank to kill off Mecha'thun, giving you a bit of flexibility to use Ghostly Strike for card draw instead or Walk the Plank for emergency removal. But if you're not under pressure, the safest road is to keep both in hand so you can kill off Anka, the Buried yourself as well.

Similarly, Preparation can be used to discount Walk the Plank or Unstable Portal.