At "Hard" difficulty level, your AI should act like a seasoned MP gamer:
Use specialised build orders
Use adaptive research paths
Use excellent droid designs and attack in groups
Use micro-management and early rushes
Basically, your AI should be playing to win the game and looking to dominate the map from the outset. Players who select this difficulty level for their AIs are looking for a strong opponent or ally.
Other great resources: Official JS API docs, Scripting Forum