Angel of Death
"Kill an alien while flying. Single player only."
This achievement requires you to have the Archangel Armor equipped on one of your squad (recommend using the Sniper). During a battle, use the armor to fly your squadmate into the air (right-trigger, then d-pad over to select the angel wings, then up on d-pad to rise), then snipe any alien with that squadmate.
Achievement will unlock as soon as you make your first kill, while flying.