Defeat 2 foes within 10 seconds in 'Insidious Alliance.'
You won't have a chance to do this on the first wave since the enemies come at you one at a time. Starting with the second wave though, you'll have multiple enemies at once. Knock the health of one enemy down below half and then concentrate on the other one. Go back and forth getting both of them pretty low. Once both are near dead, and near each other, use one of your character's powers to knock out one and then the other. See No One Left Behind for team details.