When the round starts, I switch to the cannon. I charge forward in battroid and knock out each laser thing while circling the head (sometime it's even easier to use the gunpod). When he loses his shield, blast away with the remain cannon rounds, then just circle him again (holding down R2) and firing your gunpod. To avoid the missiles, I wait till they're near, then roll forward (back then forward, quickly on the analog stick). The missiles will usually make a second pass. When they get close again, roll backwards. If you have 50-60% health, I think an S rank is within reach, of course, depending on your speed and accuracy for the first half of the mission.