To make your life easier, use "Very Easy" mode.
Bradtos will always be the 6th opponent you face in Arcade mode. He can only be damaged when he opens up and shows a crystal. He can still attack you while it is showing though.
I used Cypher against him, and I kept on pressing X to dodge to the side until he opened up. Then I quickly let in a flurry of rapid hits by holding down RT, dodging a little, and then holding RT again. He was dead within seconds.
Specineff is also a good choice for this. Just dodge until he opens, and quickly move up to him, hit him twice with LT and RT and the match is over.