Your mission on Excidium Defence Cruiser is particularly complex. The structure has a complexity to match. Look out for secret areas, as there are several Earthshakers in this cruiser.
First, you must obtain the Red Key, then complete a circuit around the rear of the cruiser (you start at the front) and then destroy two Mini-Reactors containing the Blue and Yellow keys. Blast the triggers behind the restricted access doors and destroy two more Mini-Reactors to find the boss robot.
Look out for Earthshakers, Megas, and other useful powerups here. Do not overuse Earthshakers. You will need several for Level 15.