Best way I found to do this stupid head case mindflayer first fight. 😅 

Equip the ATB first accesory and ATB boost on red 13. Equip sleep and magnify to sleep those little ****. For tifa make sure you use unbridled strength twice to get her omnistrike and rise and fall ready. For cloud putting haste on red 13 so he can re-sleep asap alongside himself and tifa is good. A synergy ability should be about ready so pop off to pressure this mindflayer. Stagger him with either cloud or tifa. Now unleash tifa's rise n' omnistrike to multiply that stagger damage and unload. Don't forget to re-sleep with red. Mindslayer should be dead or near dead. Now obliterate the little **** to put the nightmare behind us lol.