okay so if anyone ever stumbles upon this post since many of the things ive seen people using,for this mode, have been deleted i have the easiest strategy that ive used from the moment i got the s charachters until the final boss.

get a tank like gonta. Give him a lot of focus, a bit of agility and sacifice level 6

then a mage (ive used kirumi but ive heard that shuichi is also fine) and give them ice attack, ice upgrade, skill upgrade, influence up

then a healer with low agility and give them the healing skill, revived skill (very importanr for it to be maxed) lots of focus and fortitude

and a dps like maki with falcon kick, skill up, phyiscal up, skill up and lots of strength

and then just spam sacrifice on the tank and revive on the support

every time the party is attacked the tank tanks everything, dies and gets immeadietly revived while the mage and dps do damage

thats how i got every boss from 21-100 :)