Use Bahamut- if I remember correctly Demon Gate has really high magic defense, and Bahamut ignores any type of defense. I killed it in two hits- meteorain/bahamut. Big guard is a plus as well, I recall demon gate being able to hit for around 500+. Just keep a lvl3 cure+all, give cloud a hyper, and try to get bahamut to level 2. I found this fight to be easier than the lost number fight, actually.