Just go do some levelling up. If your stuck at Seymour flux on Mt Gagazet, then just go back on Gagazet trail, and get everyone up a few sphere levels. Once you level up enough, he's a wimp.

Honestly, this guy is just an annoying little puss. Cast Bio on him, and he goes "ah!" and losses 1400hp everytime he makes a move. And if you cast hastega (or haste) on your party, he doesn't have much time to use lance of atrophy that often, so you will have time to use a holy water before he uses Full-Life (which kills you instantly when inflicted with zombie).

I just beat him last night, and I had to be careful not to kill him too quickly, so everyone could get a shot in and get exp.