He may kill you fast, but he has about as much health as a piece of wet tissue paper. You should be able to take him out with 3 combos if at least one of them is after you steal his keyblades.
Block his first attack and do a full combo. Proceed to dodge his next 4 attacks and complete the reaction command. Dodge through the lasers and do a full combo. Keep dodging his attacks with your back to him to trigger his second reaction command. After doing it combo him one last time. If you do this he shouldn't even get a chance to use his DM.
Thanks for the info.
I've died too many times to him now and I'm super pissed off, so I'm going to grind some more.