Strategy and tactics games have developed some common features. One is the Attack-to-Move command. It makes units move, then stop as soon as they have an enemy in range so they can attack at maximum range, then when there are no more targets in range, the unit repeats the process of move-stop-shoot.
Titans doesnt have that. Instead you have to target specific units or give a general order to move units and they fire automatically even at things that are not important such as depots.
Shift+Left mouse would be a good hotkey.