IMPORTANT: Start doing daily relic draw, 5* and above rate has been buffed from 0.01% to something like ~25%.
I beat Bizarro Sephrioth CM, it's a lot easier than you may think.
Some guy beat it on reddit with shitty SSBs with a physical team, turns out that the Wings & Core have the same amount of health as the Body, so you may as well just ignore the Wings/Core and attack the body even though you do half damage. Killing the Wings doesn't reduce damage, it just gives the Core & Body twice as many turns and the Body has the AoE attack. Killing the Head reduces damage from the Body but if you do it three times it AOE dispels you.
So this is how you do it:
1) Aeris with Hastega/Curada and any of her SBs. Reddit guy used her SSB. I used USB1 then BSB1 when I ran out of Curada. You can probably do it without Hastega (I didn't need it) if you trust slow resist assessories RNG, bringing a second heal.
2) Cith Sith with magic debuff dances, SB optional. Lots of combos here, I used Enfeebling Jitterbug/Heathen Frolic Sarabande/SSB. Boss does 1000-2000 damage tops if you do this.
3) Three physical DPS characters. Options here, depends on your SBs. Guy on reddit had Cid attack boostga SB, I had Red XIII attack boostga BSB - so a boost is probably needed unless you've got great physical DPS relics. Cloud USB1 & Omega Drive was doing ~8000x4. Tifa with Piercing Strike did 9999. Red XIII used Armour Breakdown and Full Break. Lifesiphon is physical too. Look out for USBs/BSBs that are elemental but also have a non elemental option.
4) Fire/Ice/Lightning/Earth Dampen magicites, inheritied with another Dampen. Last slot for Evrae to boost your damage. Slow resist assessories. Tyro Godwall RW.