How to Complete Stowaways
The objective is to defeat all foes. These foes are all weak to Holy. The undead enemies will turn into graves when they reach 0 HP, and those graves will turn back into the enemies after a few rounds. But if you have a unit that knows a move like Exorcise, Burial, or Sanctify, you can permanently remove the graves from the field, preventing them from coming back to "life".