But that'd be a skill challenge, with their power use providing them an alternate skill to use for the challenge.
Let's see.
Complexity 4. Sure.
Athletics (for physical labor), Diplomacy (to represent coordinating villagers), and Intimidate (to scare the villiagers into action) successes would grant a success to the challenge, failures adding a failure. Intimidate would add two failures if failed.
As well, Nature and History could be used -once- each in order to get a lay of the land or a knowledge of historical battles.
Thievery or Bluff could be used not to garner successes, but to set traps or a funnel that could be used in the oncoming battle.
Insight or Perception (only one) could be used to determine their most likely avenue of attack, granting a success from saved labor.
As well, Athletics checks could not be Aided, as two persons digging with one shovel makes no sense, and everyone's -already- 'Aiding' each other in that sense. Also, failures on Aid Other for other skills end in failure as well.
So within that framework, the Wizard could say 'Well, I'll cloud of daggers the ground to break it up, while using mage hand to move rocks out of the way. Then once the ground is broken, I'll move on, so the villagers can finish the job once the hard part's started.' I'd say 'Excellent, you can use Arcana instead of Athletics to represent your magic being used in place of labor'.
Succeeding the challenge would make the upcoming encounter of a standard difficulty, with a short rest after. Failure would mean the enemy comes with a force that -isn't- slowed down by the defenses, and the difficulty would be higher, and may even be without a short rest.