Destroy target tapped creature. You gain 1 life for each creature you control with flying.
If the target creature is an illegal target by the time Aerial Assault tries to resolve, the spell doesn’t resolve. You won’t gain life. If the target is legal but is not destroyed (most likely because it has indestructible), you will gain life.
If any creatures were exiled “until” the target creature left the battlefield, those creatures will return before you gain life.