In the cave, have no Legions active so that you can run around. Run up to the Boss and attack its right leg repeatedly. It does not matter if it is the right or left leg, but must be a front leg. After its leg falls to the ground, quickly summon a Legion (preferably Guilt) and press Force Attack (default Triangle). Attack it for one volley, then press Force Attack again and begin to get away from the Boss. Unsummon your Legion, and look at the Boss while in one of the two small clearings. While facing the Boss, hold R1 and as soon as its first laser is about to hit you, press X and movement Analog-stick Right. While still holding R1, repeat that maneuver once again. Then, repeat by attacking the leg and so on. Note: If the Boss falls from its perch, quickly get near its head and attack the now visible silver orb along by using Triangle with your Legions to assault it.