I completed this on Easy, for other difficulties the strategy may be different. Use force fire to control where the Starlight will hit. Assign a control group key to it to quickly select it if it loses focus. Learn to control the beam. It can also be turned off if it becomes a nuisance. The biggest issue with it is that your primary targets are just out of its range.
You need to move quickly. Use your Paladins to destroy some of the enemy's factories and Singularity reactors. Also, try using cheap units on the front so the DRP would hit them instead of your base. It will attempt to attack your strike force, too, so keep them on the move. HTH.
