first of all, after u beat death gaze or death something (i haven't played ffix in awhile either), go back to the previous screen and save. that way u never have to fight him again if u lose. now, kuja is relatively easy compared to necron, even tho they have the same hp.

hopefully, u have a dark matter with you, which will do 9,999 damage automatically. if not, thats ok. just make sure ur characters are immune to status effects. also, being at a high level will also help. i fought him at lever 50 something, and once i heard my friend said i needed to be at around level 70. so i leveled up, and beat necron. if i remember correctly, there's a red fiend (forgot the name) that shows up near the part in memoria with a broken clock. that gives a relatively lot of experience, or at least when i played it helped me level up.

hope i helped