Just before you talk to them to complete the quest they gather around the last orcs who died. If you have some way to cast Meteorstrike, talk to the last dialogue panel, cast the spell and quickly go back to complete the dialogue. You get the quest XP just as the first fireballs rain down. Notice that they have some fire resistance and probably won't die from the Meteorstrike alone, but should be low enough to finish them off in a couple turns.
