The Dragoon can fire ground to air or ground to ground with incredible effectivness. It works even better if the Attack range has been upgraded. If you need a big attack force early in the first Protoss campaign I suggest that you get a squad or two of Dragoons and one or two squads of Zealots. What you do is make and minor attack on say a Zerg base draw their forces out and then engage them away from the defense structures with your Zealots and annialte them with your Dragoons which if done correctly will no be touched. Dragoons are probably one of the best Protoss units to destroy air people with..