First time I fought him I didn't have any equipment on Aerith, took me like an hour and I think the game just pitied me and gave me a lucky break with my limit. GOAT game but the final gauntlet has a few poor game designs.
What's your equipment like? Aerith should be warding herself and bombing the shut out of Sephy with Firagas while Cloud circles around and slashes and everytime you get to Aerith you also drop a Firagas.
I ended up beating him. Really you need a good load out to manage that and I didn't have one. Something with haste would have worked which I had on a different party member. I didn't want to redo the entire boss gauntlet cause it was pretty long.
I had to do it over and over until I reached the perfect condition of both party members having almost full health and two ATB bars as soon as he starts the end is nigh. At that point I had the 2nd character unload both their ATB bars in the best attack possible before they died and Cloud do the same then I stayed as close as possible and kept up an assault. I delayed the limit break as long as I could.
The difference was before the AI got my 2nd character killed I used their ATBs up and didn't bother healing, just let them die after.