How to Complete Plea for Help
The objective is to defeat all foes. The enemies are all undead, so when they are "killed", they turn into graves. After a few turns, those graves turn back into the undead units. Certain moves, such as Exorcise and Burial, will permanently remove those graves from the field. You can still complete the quest by just turning all of the enemies into graves, but it can be difficult to do this quickly before any of the graves turn back into enemies.
The specific enemies in this battle are five Ghosts and a Wraith. Since they are weak to Holy, it can be helpful to have a Paladin on your team.