FFBE SBB Chaotic Darkness EXT 19 Turns 

Thanks Cloud03, almost 15 turns. if yuna ex2 or my ff a bit stronger. maybe i should get that step up vc, as i dont have 170 atk/mag vc aside the water ice earth element ones
Sorry for slow record, my phone is so hot
Turns action to be noted:
For EX2 DPS: Turn 1, 6, 15 Buff LB, Area Effect, etc
For EX3 FF: Turn 2 Buff LB, Turn 6 Yuna cast Bracer to FF, Turn 11 Buff LB
Xon Projected Mirror: Turn 3, 7, 11, 15, to achieve this need Melissa LB to Xon
Xon Copy: Turn 1 for Human Killer, Turn 3 for Demon Killer then spread it, Turn 5 copy someone, Turn 10 copy again, Turn 14 copy again
Use Melissa Bardarkja & Celeste Barlightja to increase LB gauge so FF can spam LB, Melissa Second of Support also good
Use Yuna summon Bahamut for Area Effect imperil
NOTE THAT BOSS WILL HEAL UNDER THRESHOLD 50%, FOR 10% HP
so please be prepared for 33B HP boss
BUT THEN BOSS WILL HEAL AGAIN UNDER THRESHOLD 20%, FOR 10% HP PER TURN FOR 2 TURNS
so a wise choice is to wait until you can burst that 20%
As passing the threshold boss will just use graviga, but will heal on the next turn. So before 20% prepare to kill in 2 turns (turn 13 leave Boss HP 21% above, burst on turn 14 and 15 to kill)