Scorpios can only target and shoot one air troop at a time.
It cannot harm ground-based units, or interfere with them in any way other than potentially distracting them from attacking a more valuable target (e.g. the Town Hall).
The Scorpio has an excellent range and a very high damage per shot. As the in-game description states, it's best to position them so that they cover as much area as possible.
The Scorpio is a popular first target for attackers via ground troops as they are the only defensive structure that cannot target them. Make sure they are defended well, and they will help to keep the skies clear from rampaging Zeppelins, Dutchmen and Vertibirds. Scorpions are there for when Gun Turrets just won't cut it (and they usually don't, for air units at least).
If possible, keep your Scorpions behind your first lines of defense (i.e. behind Guns and Gun Turrets), preferably with a Wall in between. This will prevent a large Brute assault from quickly destroying the Scorpion and allowing a Vertibird to be deployed early in the battle. The longer it takes the Brutes to destroy the Scorpios, the more time your turrets will have to thin out their ranks before the Vertibird is deployed.
The Scorpio is a natural target to attack first, as clearing them helps pave the way for an air assault. Vertibirds are very vulnerable to the Scorpios so it is a good strategic move to destroy this building with Brutes before deploying her. They are easily destroyed by any ground-based troop type.
It is advisable to keep Zeppelins away from a Scorpio, as they are capable of destroying Zeppelins in one or two hits from a great distance.
Although Scorpios are quite effective against Dutchmen, it is possible to keep a Scorpion occupied with a Dutchman or two long enough for a fleet of Zeppelins to quickly destroy a majority of the ground defenses.