By god I am probably the first one in this world to do this. Before I delve deep into this I'll just say that every phase is a separate run as this is fully spliced with many retries and different builds. I eagerly await the day that someone does all of this in one run. Now let's get to it.

So we get the Cloud Zack portion first and it's not too bad, some fun stuff you can pull off but in order to pressure Sephiroth you need to hit a "heavy" attack in Sephiroth's openings which is from Cloud's Punisher Counter back into Operator Mode slash or most abilities and the unique Counterfire you have with Zack here. I just wanted to flex with showing off the Synergy Attack which unintentionally had the sickest dodge from Sephiroth's Skewer, and letting Zack limit clash with Sephiroth.

Reborn phase 1 and 2 are not that bad but you need to have limit in order to avoid Heartless Angel.

Whisper Bahamut Arisen might be tricky cuz of the near instant Gigaflare but since he aggros onto one party member instantly at the start just let them wail on them with HP-MP swap materia to get limit and boom easy stagger to take out the wings afterwards.

Phase 4 with Zack solo arguably the easiest so nothing to say but for some reason I did not get 3 ATB with the Synergy Ability here. Weird.

Phase 5 aka final phase of Reborn EASILY the hardest since you NEED to prepare hard for this beforehand. The worst problem with the final boss rush is trying to have the right party depending on your affinity with party members and whoever you date. Here Barret has limit carried over from another attempt with the earlier Bahamut fight and Red has limit from Götterdämmerung which I kind of didn't want to do since Yuffie needed to steal the limit from him but there can be other ways around this. Yuffie kills the first wing hard with Swiftcast Magic Focused Firagas and her Magic Ninjutsu kills the other wing. Red's Sentinal Stance is godlike to avoid taking damage from the wings physical attack Atonement, and if you time Ninja Bazooka good with Yuffie you can avoid damage with her and another party member. Then kill the core fast via stunlocking with tier 3 magic and Synergy Attacks. Heartless Angel the BIG obstacle here since it is only avoidable with Limits but Red going into Vengeance makes him invincible to it which is cool. Jump materia can also most likely avoid it too. Once you stagger should be an easy kill.

Final fight with Cloud Aerith and Sephiroth at the end one of the best fights in this game, very fun but he also has Heartless Angel here so be prepared. But yeah what a gauntlet and I'm very happy I was able to do all of this. Good luck to people trying to do this in the future.