One big blast from his cannon mode could win this for Shockwave. However, Bruticus has at least some intelligence in battle. He would try to take S by surprise, perhaps by bombarding him in his laboratory when he thinks that he's safe. It's possible that B can knock out S with powerful shelling and other weapons, especially the X-ray laser, glue gun and gyro gun. S is more experienced, though. He'd be able to escape, transform, fly to a new location and unleash his deadly bolts on the slower combiner.
Unless B kills S quickly, the big purple gun will wreck him like a badly designed Russian tank.
[B S:
]