Lightning is a good offensive spell with the highest elixir cost (6) similar to Rocket. It damages the 3 highest health in the target area. It usually used to counter Giants, Witch, Wizard and Three Musketeers. It can also deal good amount of damage to the Arena Towers.
Another good counterpart to the Lightning spell is the Rocket spell. They both have high damage but the Rocket will damage any units within its area, while Lightning chooses the three high health units.