Yedora triggers, returning it to the battlefield face-down as a Forest land.
Tap that Forest for ({G} magic symbol) .
Activate Primordial Mist by exiling the face-down Walking Ballista, and allowing you to cast it this turn.
Repeat.
Once you have infinite ({G} magic symbol) , you may cast Walking Ballista using an arbitrarily large amount of mana, and then use its second ability to deal an arbitrarily large amount of damage to any target(s).